As filed with the Securities and Exchange Commission on November 15, 20162022

RegistrationNo. 333-          

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM S-4

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

ARTHUR J. GALLAGHER & CO.

(Exact name of registrant as specified in its charter)

Two Pierce Place

Itasca,2850 Golf Road, Rolling Meadows, Illinois

60143-314160008-4050

(630) 773-3800

(Address, including zip code, and telephone number, including area code, of registrant’s principal executive offices)

Walter D. Bay, Esq.

Vice President, General Counsel and Secretary

Arthur J. Gallagher & Co.

Two Pierce Place

Itasca,2850 Golf Road, Rolling Meadows, Illinois

60143-314160008-4050

(630) 773-3800

(Name, address, including zip code and telephone number, including area code, of agent for service)

COPIES TO:

Andrew L. Fabens, Esq.

Gibson, Dunn & Crutcher LLP

200 Park Avenue

New York, New York 10166

(212) 351-4034

Approximate date of commencement of proposed sale of the securities to the public: From time to time after this registration statement becomes effective.

If the securities being registered on this Form are being offered in connection with the formation of a holding company and there is compliance with General Instruction G, check the following box.  ☐

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☐

If this Form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☐

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer” andfiler,” “smaller reporting company” and “emerging growth company” inRule 12b-2 of the Exchange Act.

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act.  ☐

If applicable, place an X in the box to designate the appropriate rule provision relied upon in conducting this transaction:

Exchange ActRule 13e-4(i) (Cross-Border Issuer Tender Offer)  ☐

Exchange ActRule 14d-1(d) (Cross-Border Third-Party Tender Offer)  ☐

CALCULATION OF REGISTRATION FEE

The registrant hereby amends this registration statement on such date or dates as may be necessary to delay its effective date until the registrant shall file a further amendment which specifically states that this registration statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933, as amended, or until this registration statement shall become effective on such date as the Securities and Exchange Commission, acting pursuant to said Section 8(a), may determine.

The information in this prospectus is not complete and may be changed. We may not sell these securities until the registration statement filed with the Securities and Exchange Commission is effective. This prospectus is not an offer to sell these securities and it is not soliciting an offer to buy these securities in any state where the offer or sale is not permitted.

SUBJECT TO COMPLETION, DATED NOVEMBER 15, 20162022

PROSPECTUS

10,000,000

7,000,000 Shares of Common Stock

This prospectus relates to 10,000,0007,000,000 shares of our common stock, par value $1.00 per share, that we may offer and issue from time to time in connection with future acquisitions of other businesses, assets or securities by us.

We will determine the amount and type of consideration to be offered and the other specific terms of each acquisition following negotiation by us with the owners or controlling persons of the businesses, assets or securities to be acquired. The consideration for any such acquisition may consist of shares of our common stock or a combination of common stock, cash, notes or assumption of liabilities. We may structure business acquisitions in a variety of ways, including acquiring stock, other equity interests or assets of the acquired business or merging the acquired business with us (including one of our subsidiaries). We expect that the shares of common stock issued in connection with these transactions will be valued at a price reasonably related to the market value of our common stock either at the time an agreement is reached regarding the terms of the acquisition, at the time we issue the shares, or during some other negotiated period. Persons to whom we issue our common stock under this prospectus may also use this prospectus to resell the common stock. We have not fixed a period of time during which the common stock offered by this prospectus may be offered or sold.

We may also issue shares of common stock upon the exercise of options, warrants, convertible securities or other similar securities assumed or issued by us from time to time in connection with these transactions.

We will pay all expenses of this offering. We will not pay underwriting discounts or commissions in connection with issuing these shares, although we may pay finder’s fees in specific acquisitions. Any person receiving a finder’s fee may be deemed an “underwriter” within the meaning of the Securities Act of 1933, as amended.

Our common stock is traded on the New York Stock Exchange under the symbol “AJG.” On November 14, 2016,2022 the last reported per share sale price of our common stock was $52.12.$185.77.

Investing in our common stock involves risk. You should carefully consider the “Risk Factors” described on page 3 in determining whether to accept stock as all or part of the purchase price for our acquisition of your business, securities or other assets.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus. Any representation to the contrary is a criminal offense.

The date of this prospectus is                    .

TABLE OF CONTENTS

CONTENTS

ABOUT THIS PROSPECTUS

This prospectus is part of a “shelf” registration statement onForm S-4 that we filed with the Securities and Exchange Commission (“SEC”). Under the shelf registration process, we may from time to time offer and issue up to 10,000,0007,000,000 shares of our common stock in connection with future acquisitions of other businesses, assets or securities. This prospectus includes (through incorporation by reference) a general description of the common stock that we may offer and issue. We may add, update or change the information contained in this prospectus by means of one or more prospectus supplements. Before investing in our common stock, please carefully review both this prospectus and any prospectus supplement, together with the additional information described under the heading “Where You Can Find More Information” of this prospectus.

This prospectus incorporates important business and financial information about us that is not included in or delivered with this prospectus. We will provide, without charge, a copy of any or all of the documents incorporated by reference in this prospectus upon written or oral request. Direct any such requests to: General Counsel, Arthur J. Gallagher & Co., Two Pierce Place, Itasca,2850 Golf Road, Rolling Meadows, Illinois60143-314160008-4050 (telephone number(630) 773-3800).To obtain timely delivery, you must request the information no later than five business days before the date that you must make your investment decision.decision.

You should rely only on information contained in or incorporated by reference in this prospectus. We have not authorized anyone to give you any information or make any representation that is different from, or in addition to, that contained in this prospectus or in any of the materials that we have incorporated by reference into this document. If anyone does give you information of this sort, you should not rely on it. If you are in a jurisdiction where offers to sell, or solicitations of offers to purchase, the securities offered by this document are unlawful, or if you are a person to whom it is unlawful to direct these types of activities, then the offer presented in this document does not extend to you. The information contained in this prospectus (including the information contained in any document incorporated by reference in this prospectus) speaks only as of the date of each such document, unless the information specifically indicates that another date applies.

ARTHUR J. GALLAGHER & CO.

We are engaged in providing insurance brokerage and consulting services, and third-party property/casualty claims settlement and administration services to entities in the U.S. and abroad. Throughout 2015 and into 2016,In the nine-month period ended September 30, 2022, we have expanded, and expect to continue to expand, our international operations through both acquisitions and organic growth. We generated approximately 68%65% of our revenues for the combined brokerage and risk management segments domestically with the remaining 32% derivedand 35% internationally, primarily in Australia, Bermuda, Canada, the Caribbean, New Zealand and the U.K. (based on second quarter 2016 revenues).

We have three reportable segments: brokerage, risk management and corporate, which contributed approximately 63%86%, 12%14% and 25%,0% for brokerage, risk management and corporate, respectively, to revenues during the nine-month period ended September 30, 2016.2022. Our major sources of operating revenues are commissions, fees and supplemental and contingent commissionsrevenues from brokerage operations and fees from risk management operations. Investment income is generated from invested cash and fiduciary funds, clean energy and other investments, and interest income from premium financing.

For more information about our business, please refer to the “Business” section in our most recent Annual Report on Form 10-K filed with the SEC and incorporated by reference in this prospectus and the “Management’s Discussion and Analysis of Financial Condition and Results of Operations” sections of our most recent Annual Report on Form 10-K and all subsequent Quarterly Reports on Form 10-Q filed with the SEC and incorporated by reference in this prospectus.

Shares of our common stock are traded on the New York Stock Exchange under the symbol AJG.“AJG”. We were reincorporated as a Delaware corporation in 1972. Our executive offices are located at Two Pierce Place, Itasca,2850 Golf Road, Rolling Meadows, Illinois 60143-3141,60008-4050 and our telephone number is (630) 773-3800.

Unless the context otherwise requires, the terms “we,” “our”, “Gallagher”, and “us” refer to Arthur J. Gallagher & Co. and/or its subsidiaries, as appropriate. The term “you” refers to a prospective investor.

RISK FACTORS

Investing in our common stock involves risks. Before making an investment decision, you should carefully consider the specific risks set forth under the caption “Risk Factors” in the applicable prospectus supplement as well as under the caption “Risk Factors” in our filings with the SEC pursuant to Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act of 1934, as amended (the “Exchange Act”), which are incorporated by reference into this prospectus.prospectus, including our Annual Report on Form 10-K for the fiscal year ended December 31, 2021. Our business is influenced by many factors that are difficult to predict and beyond our control and that involve uncertainties that may materially affect our results of operations, financial condition or cash flows, or the value of our common stock. If any one or more of the risks set forth in the applicable prospectus supplement or documents incorporated by reference into this prospectus actually occur, our business, financial condition or results of operations would likely suffer. In addition, the trading price of our common stock could decline, and you may lose all or part of your investment in our common stock.

CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS

This prospectus and the information incorporated by reference in this prospectus contain certain statements related to future results, or statestates our intentions, beliefs and expectations or predictions for the future, which are forward-looking statements as that term is defined in the Private Securities Litigation Reform Act of 1995. Forward-looking statements relate to expectations or forecasts of future events. Such statements use words such as “anticipate,” “believe,” “estimate,” “expect,” “contemplate,” “forecast,” “project,” “intend,” “plan,” “potential,” and other similar terms, and future or conditional tense verbs like “could,” “may,” “might,” “see,” “should,” “will” and “would.” You can also identify forward-looking statements by the fact that they do not relate strictly to historical or current facts. For example, we may use forward-looking statements when addressing topics such as: comments regarding the expected benefits of our acquisition of the Willis Towers Watson plc treaty reinsurance brokerage operations (the “Willis Re acquisition”); the remaining international deferred closing that requires local regulatory approval; the expected duration and costs of integration of such acquisition; the impact of Russia’s invasion of Ukraine; market and industry conditions, including competitive and pricing trends; acquisition strategy;strategy including the expected size of our acquisition program; the expected impact of acquisitions and dispositions; the development and performance of our services and products; changes in the composition or level of our revenues or earnings; our cost structure and the size and outcome of cost-saving or restructuring initiatives; future capital expenditures; future debt levels;levels and anticipated actions to be taken in connection with maturing debt; future debt to earnings ratios; the outcome of contingencies; dividend policy; pension obligations; cash flow and liquidity; capital structure and financial losses; future actions by regulators; the outcome of existing regulatory actions, investigations, audits, reviews or litigation; the impact of changes in accounting rules; financial markets; interest rates; foreign exchange rates; matters relating to our operations; income taxes; expectations regarding our investments,investments; human capital management, including our clean energy investments; the financial impact of retention agreements in our international brokerage operations;diversity and inclusion initiatives; environmental, social and governance matters, including climate-resilience products and services and carbon emissions; and integrating recent acquisitions. These forward-looking statements are subject to certain risks and uncertainties that could cause actual results to differ materially from either historical or anticipated results depending on a variety of factors.

Potential factors related to the Willis Re acquisition that could impact results include:

Additional potential factors that could impact results include:

A detailed discussion of the factors that could cause actual results to differ materially from our published expectations is contained under the heading “Risk Factors” in our filings with the SEC, including our Annual Report onForm 10-K for the fiscal year ended December 31, 20152021 and our Quarterly Reports on Form 10-Q for the quarterly periods ended March 31, 2016,2022, June 30, 20162022 and September 30, 2016,2022, and any other reports we file with the SEC in the future, which are incorporated by reference into this prospectus.

ACQUISITION PROGRAM AND PLAN OF DISTRIBUTION

The shares of our common stock covered by this prospectus may be offered by us from time to time in connection with future acquisitions of other businesses, assets or securities. We are actively looking for acquisition opportunities in the insurance brokerage and employee benefits consulting markets. We typically seek targets with a strong middle market client focus or significant expertise in one of our focus market areas. Acquisition targets have typically been within a $1,000,000 to $50,000,000$100,000,000 purchase price range, although we may complete acquisitions with a higher purchase price. From time to time, we also consider acquisitions in our reinsurance brokerage businesses or our risk management segment, consistingwhich consists of claim settlement and administrative service providers. In recent years, many of our acquisitions have occurred outside the United States.

We expect to offer and sell the shares covered by this prospectus in connection with future acquisitions within the next two years. We anticipate that our future acquisitions will consist principally of additional insurance brokerage, employee benefits consulting and related businesses. The consideration for such acquisitions may include cash, including installment payments, earnout payments, shares of common stock, evidences of debt, other securities including securities that may be converted into common stock, guarantees, assumptions of liabilities, or any two or more of the foregoing, as determined from time to time by negotiations between us and the owners or controlling persons of the businesses, assets or securities to be acquired. In addition, we may enter into employment contracts and non-competition agreements with former owners and key executive personnel of acquired businesses. At any given time we are typically engaged in preliminary discussions or negotiations with many candidates regarding possible future acquisitions.

In general, the terms of each future acquisition will be determined by negotiations between our representatives and the owners or controlling persons of the businesses, assets or securities to be acquired. The factors taken into account in determining the terms of an acquisition may include the established quality and reputation of the business to be acquired and its management, its gross commission revenues, earning power, cash flow, growth potential, the location of the business and assets to be acquired, its expertise in an attractive niche, and the geographical and service diversification we anticipate as a result of the acquisition. We anticipate that shares of our common stock issued in any future acquisition will be valued at a price reasonably related to

the then current market value of our common stock as reported on the New York Stock Exchange at or about the time or times of delivery of the shares. We do not expect to receive any cash proceeds, other than cash balances of acquired companies maintained in the ordinary course of business, in connection with any such issuances.

This prospectus may be supplemented to furnish the information necessary for a particular negotiated transaction, and the registration statement of which this prospectus is a part will be amended or supplemented, as required, to supply information concerning an acquisition.

We will pay all expenses of any offerings under this prospectus. We do not expect that underwriting discounts or commissions will be paid by us except that finder’s fees may be paid to persons from time to time in connection with specific acquisitions. Any person receiving any such fees may be deemed to be an “underwriter” within the meaning of the Securities Act of 1933, as amended (the “Securities Act”).

DESCRIPTION OF CAPITALCOMMON STOCK

General

The following summary description sets forth some of the general terms and provisions of our common stock. Because this is a summary description, it does not contain all of the information that may be important to you. For a more detailed description of our capitalthe common stock, is only a summary. For more complete information, you should refer to our amended and restated certificate of incorporation, bylaws and any amendments thereto, which we have filed with the SEC. In addition, you should refer to the Delaware General Corporation Law,provisions of our Amended and Restated Certificate of Incorporation (the “Certificate of Incorporation”) and our Amended and Restated By-Laws (the “By-Laws”), each of which also governsis an exhibit to our structure, management and activities.most recent Annual Report on Form 10-K filed with the SEC.

Common StockGeneral

Under our amended and restated certificatethe Certificate of incorporation, our board of directors, which we refer to as our board,Incorporation, the Company is authorized to issue up to 400,000,000 shares of common stock with par value of $1.00 per share and up to 1,000,000 shares of preferred stock with no par value per share. The shares of common stock currently outstanding are fully paid and nonassessable. As of September 30, 2022, there were 210,840,399 shares of common stock issued and outstanding. No shares of preferred stock are currently outstanding.

No Preemptive, Redemption or Conversion Rights

Our common stock is not subject to redemption or retirement, is not subject to sinking fund provisions, does not have any conversion rights and is not subject to call. No holder of our common stock has preemptive or other rights to subscribe for additional shares of any class of our stock.

Voting Rights

Each holder of our common stock is entitled to one vote for each share of such stock standing in his or her name on the books of the Company. Holders of shares of our common stock do not have cumulative voting rights in the election of directors.

Board of Directors

Our Board of Directors is not classified. Our Certificate of Incorporation establishes that the number of directors shall not be less than three nor more than fifteen, with the exact number of directors to be fixed from time to time by, or in the manner provided in, the By-Laws. Our By-Laws provide that, within such limits, the number of directors shall be determined by resolution of the Board of Directors.

No Action by Stockholder Consent

The Certificate of Incorporation provides that any action required or permitted to be taken by the stockholders must be taken at a duly called annual or special meeting of the stockholders, and the power of stockholders to consent in writing, without a meeting, to the taking of any action is specifically denied.

Power to Call Special Stockholder Meeting

Under Delaware law, a special meeting of stockholders may be called by our Board of Directors or by any other person authorized to do so in the Certificate of Incorporation or By-Laws. Pursuant to our By-Laws, special meetings of the stockholders may be called by the chairman of our Board of Directors or President. In addition, a special meeting of the stockholders shall be called by the President or Secretary at the request in writing of a majority of the Board of Directors.

Dividend Rights

The holders of our common stock will beare entitled to receive such dividends as our boardthe Board of Directors may declare from time to time, provided that any and all preferred dividends on our preferred stock for the then

current quarter have been set aside or paid, and all prior quarterly dividends on our preferred stock have been paid in full.

Rights upon Liquidation

Upon our liquidation, the holders of our common stock will receive ratably, in proportion to the number of shares held, all of our net assets remaining after the payment of any liquidation preference payable with respect to any preferred stock that may then be outstanding. Our common stock is not subject to redemption or retirement. Each holder of our common stock is entitled to one vote for each share of such stock standing in his or her name on the books of the corporation. The holders of our common stock do not have cumulative voting rights in the election of directors. No holder of our common stock has preemptive or other rights to subscribe for additional shares of any class of our stock.

As of November 11, 2016, there were 178,112,404 shares of common stock issued and outstanding.

Preferred StockForum Selection Clause

Under our amendedBy-Laws, unless the Company, in writing, selects or consents to the selection of an alternative forum, (a) the sole and restated certificateexclusive forum for any complaint asserting any internal corporate claims (as defined below), to the fullest extent permitted by law, and subject to applicable jurisdictional requirements, shall be the Court of incorporation,Chancery of the State of Delaware (or, if the Court of Chancery does not have, or declines to accept, jurisdiction, another state court or a federal court located within the State of Delaware); and (b) the sole and exclusive forum for any complaint asserting a cause of action arising under the Securities Act of 1933, to the fullest extent permitted by law, shall be the federal district courts of the United States of America. “Internal corporate” claims means claims, including claims in the right of the Company: (A) that are based upon a violation of a duty by a current or former director, officer, employee or stockholder in such capacity, or (B) as to which the General Corporation Law of the State of Delaware confers jurisdiction upon the Delaware Court of Chancery. In addition, under our board is authorizedBy-Laws, to issue up to 1,000,000the fullest extent permitted by law, any person or entity purchasing or otherwise acquiring or holding any interest in shares of preferred stock. the Company’s shall be deemed to have notice of and consented to these forum selection provisions.

Preferred Stock

Our preferred stock may be issued in one or more series, and for such consideration as our boardBoard of Directors may determine. Our boardBoard of Directors is authorized to determine the voting power of each series of preferred stock, which may range from no voting power to a maximum of one vote per share. If our boardBoard of Directors does not explicitly provide the voting power of any series of our preferred stock in the resolution or resolutions providing for the issuance of such series, the holders of that series of preferred stock have no voting power with respect to any matter. Our boardBoard of Directors is also authorized to fix the designations, preferences qualifications, limitations, restrictions and relative, participating, optional or other special rights, and qualifications, limitations or restrictions thereof, as provided for in the resolution or resolutions providing for the issuance of such shares of preferred stock.

All shares of one series of preferred stock must be of equal rank and identical in all respects. No dividend may be paid or declared on any particular series of preferred stock unless dividends are to be paid or declared pro rata on all shares of preferred stock that rank equally as to dividends with such particular series, and are outstanding at such time.

LEGAL MATTERS

Seth Diehl, Esq., our SeniorAssociate General Counsel Corporate– Securities & Securities,Governance, has rendered an opinion with respect to the validity of the securities being offered by this prospectus.

EXPERTS

The consolidated financial statements and related financial statement schedule of Arthur J. Gallagher & Co. appearing in ourArthur J. Gallagher & Co.’s Annual Report onForm 10-K(Form 10-K) for the fiscal year ended December 31, 2015 (including the schedule appearing therein),2021, and the effectiveness of ourArthur J. Gallagher & Co.’s internal control over financial reporting as of December 31, 20152021, have been audited by Ernst & Young LLP, independent registered public accounting firm, as set forth in their reports thereon, included therein, and incorporated herein by reference. Such consolidated financial statements and related financial statement schedule are incorporated herein by reference in reliance upon such reports given on the authority of such firm as experts in accounting and auditing.

With respect to the unaudited consolidated interimThe combined financial informationstatements of Arthur J. Gallagher & Co. for the three-month periods ended March 31, 2016 and March 31, 2015; the three-month and six-month periods ended June 30, 2016 and June 30, 2015 and the three-month and nine-month periods ended September 30, 2016 and September 30, 2015, incorporated by reference in this Prospectus, Ernst & Young LLP reported that they have applied limited procedures in accordance with professional standards for a review of such information. However, their separate reports dated May 2, 2016; July 29, 2016 and October 28, 2016, includedWillis Re DivestCo appearing in Arthur J. Gallagher & Co.’s QuarterlyCo’s Current Report on Form 10-Q for the quarters ended March 31, 2016, June 30, 2016 and September 30, 2016, respectively, and8-K/A dated February 14, 2022 have been audited by Deloitte & Touche LLP, an independent auditor, as stated in their report included therein. Such financial statements are incorporated herein by reference herein, states that they did not auditin reliance upon the report of such firm given their authority as experts in accounting and they do not express an opinion on that interim financial information. Accordingly, the degree of reliance on their report on such information should be restricted in light of the limited nature of the review procedures applied. Ernst & Young LLP is not subject to the liability provisions of Section 11 of the Securities Act of 1933 (the “Act”) for their report on the unaudited interim financial information because that report is not a “report” or a “part” of the Registration Statement prepared or certified by Ernst & Young LLP within the meaning of Sections 7 and 11 of the Act.auditing.

WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and current reports, proxy statements and other information with the SEC under the Exchange Act. You may access and read our SEC filings, including the complete registration statement of which this prospectus is a part and all of the exhibits to it, through commercial document retrieval services, which are available to the public, and also on the SEC’s website located athttp://www.sec.gov.www.sec.gov. This site contains reports and other information that we file electronically with the SEC. This registration statement, our other filings or other information can be inspected, and copies may be obtained, at the SEC’s Public Reference Room, 100 F Street, N.E., Washington, DC 20549. Information on the operation of the Public Reference Room of the SEC may be obtained by calling the SEC at1-800-SEC-0330.

We have filed a registration statement, of which this prospectus is a part, and related exhibits with the SEC under the Securities Act. This prospectus, filed as part of the registration statement, does not contain all of the information set forth in the registration statement and its exhibits and schedules, portions of which have been omitted as permitted by the rules and regulations of the SEC. For further information about us and our common stock, we refer you to the registration statement and its exhibits and schedules.

Statements in this prospectus about the contents of any contract, agreement or other documents are not necessarily complete and, in each instance, we refer you to the copy of such contract, agreement or document filed as an exhibit to the registration statement, with each such statement being qualified in all respects by reference to the document to which it refers.

INCORPORATION OF CERTAIN INFORMATION BY REFERENCE

THIS PROSPECTUS INCORPORATES DOCUMENTS BY REFERENCE THAT ARE NOT PRESENTED IN OR DELIVERED WITH THIS PROSPECTUS. YOU SHOULD RELY ONLY ON THE INFORMATION CONTAINED IN THIS PROSPECTUS AND IN THE DOCUMENTS THAT WE HAVE INCORPORATED BY REFERENCE INTO THIS PROSPECTUS. WE HAVE NOT AUTHORIZED ANYONE TO PROVIDE YOU WITH INFORMATION THAT IS DIFFERENT FROM OR IN ADDITION TO THE INFORMATION CONTAINED IN THIS PROSPECTUS AND INCORPORATED BY REFERENCE INTO THIS PROSPECTUS.

We incorporate information into this prospectus by reference, which means that we can disclose important information to you by referring you to those documents, which have been separately filed with the SEC. The information incorporated by reference is considered to be part of this prospectus, and information that we file later with the SEC will automatically update and supersede this information. We incorporateare incorporating by reference the documentsour filings listed below and allany additional documents subsequently filedthat we may file with the SEC pursuant to SectionsSection 13(a), 13(c), 14 or 15(d) of the Exchange Act on or after the date of this prospectus and prior tobefore the date this offering is terminated or we issue alltermination of the sharesoffering of common stockthe securities made under this prospectus:

Nothing in this prospectus shall be deemed to incorporateany information furnished but(but not filed, with the SEC pursuant tofiled) under Item 2.02 or Item 7.01 of any Current Report on Form 8-K andor corresponding information furnished under Item 9.01 ofForm 8-K (or included as an exhibit) in any past or future current report on Form 8-K that we may file with the SEC),SEC, unless otherwise specified in such current report or in a particular prospectus supplement.specifically noted below:

Information in this prospectus supersedes related information in the documents listed above and information in subsequently filed documents supersedes related information in both this prospectus and the incorporated documents.

You may request orally or in writing, and we will provide you with, a copy of these filings, at no cost, by calling us at(630) 773-3800 or by writing to us at the following address:

General Counsel

Arthur J. Gallagher & Co.

Two Pierce Place2850 Golf Road

Itasca,Rolling Meadows, Illinois60143-314160008-4050

In order to ensure timely delivery of the information incorporated by reference in this prospectus, any request should be made no later than five business days prior to the date on which you plan to make a final investment decision. These filings and reports can also be found on our investor relations website, located athttp: https://www.ajg.com, by following the links to “Investor Relations” and “SEC Filings.”investor.ajg.com. Additionally, you may obtain copies of any of these through the SEC or through the SEC’s website:http://www.sec.gov.www.sec.gov.

The information contained on our website does not constitute a part of this prospectus.

10,000,000

7,000,000 Shares of Common Stock

PROSPECTUS

[                    ], 2016

                 , 2022

We have not authorized any dealer, salesperson or other person to give any information or represent anything to you other than the information contained in this prospectus. You may not rely on unauthorized information or representations.

This prospectus does not offer to sell or ask for offers to buy any of the securities in any jurisdiction where it is unlawful, where the person making the offer is not qualified to do so, or to any person who cannot legally be offered the securities.

The information in this prospectus is current only as of the date on its cover, and may change after that date. For any time after the cover date of this prospectus, we do not represent that our affairs are the same as described or that the information in this prospectus is correct, nor do we imply those things by delivering this prospectus or selling securities to you.

PART II

INFORMATION NOT REQUIRED IN PROSPECTUS

Item 20. Indemnification of Directors and Officers.

Gallagher is incorporated under the Delaware General Corporation Law (the “DGCL”).

Section 145(a) of the DGCL provides that a Delaware corporation shall have the power to indemnify any person who was or is a party or is threatened to be made a party to any threatened, pending or completed action, suit or proceeding, whether civil, criminal, administrative or investigative (other than an action by or in the right of the corporation) by reason of the fact that the person is or was a director, officer, employee or agent of the corporation, or is or was serving at the request of the corporation as a director, officer, employee or agent of another corporation, partnership, joint venture, trust or other enterprise, against expenses (including attorneys’ fees), judgments, fines and amounts paid in settlement actually and reasonably incurred by the person in connection with such action, suit or proceeding if the person acted in good faith and in a manner the person reasonably believed to be in or not opposed to the best interests of the corporation, and, with respect to any criminal action or proceeding, had no reasonable cause to believe the person’s conduct was unlawful.

Section 145(b) of the DGCL provides that a Delaware corporation shall have the power to indemnify any person who was or is a party or is threatened to be made a party to any threatened, pending or completed action or suit by or in the right of the corporation to procure a judgment in its favor by reason of the fact that such person acted in any of the capacities set forth above, against expenses (including attorneys’ fees) actually and reasonably incurred by the person in connection with the defense or settlement of such action or suit if the person acted under standards similar to those discussed above, except that no indemnification shall be made in respect of any claim, issue or matter as to which such person shall have been adjudged to be liable to the corporation unless and only to the extent that the Court of Chancery or the court in which such action or suit was brought shall determine that despite the adjudication of liability, but in view of all circumstances of the case, such person is fairly and reasonably entitled to be indemnified for such expenses which the Court of Chancery or such other court shall deem proper.

Section 145 of the DGCL further provides that to the extent a director or officer of a corporation has been successful in the defense of any action, suit or proceeding referred to in subsections (a) and (b) of Section 145 of the DGCL or in the defense of any claim, issue or matter therein, such person shall be indemnified against expenses (including attorneys’ fees) actually and reasonably incurred by such person in connection therewith; and that indemnification provided for by Section 145 of the DGCL shall not be deemed exclusive of any other rights to which the indemnified party may be entitled; and that the corporation shall have power to purchase and maintain insurance on behalf of a director or officer of the corporation against any liability asserted against such person and incurred by such person in any such capacity or arising out of such person’s status as such whether or not the corporation would have the power to indemnify such person against such liability under Section 145 of the DGCL.

Section 102(b)(7) of the DGCL provides that a corporation’s certificate of incorporation may provide a provision that eliminates or limits the personal liability of a director to the corporation or its stockholders for monetary damages for breach of fiduciary duty as a director, provided that such provisions shall not eliminate or limit the liability of a director (1) for any breach of the director’s duty of loyalty to the corporation or its stockholders; (2) for acts or omissions not in good faith or that involve intentional misconduct or a knowing violation of law; (3) under Section 174 of the DGCL; or (4) for any transaction from which the director derived an improper personal benefit. No such provision shall eliminate or limit the liability of a director for any act or omission occurring before the date when such provision becomes effective.

Article Seven of Gallagher’s Amended and Restated By-laws provide for the indemnification of each of Gallagher’s directors, officers, employees or agents to the full extent permitted by applicable law, including, without limitation, the DGCL.

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Article Seven of Gallagher’s Amended and Restated By-laws provides that Gallagher shall indemnify any person in connection with any action, suit or proceeding brought or threatened by reason of the fact that he or she is or was one of Gallagher’s directors, officers, employees or agents, or is or was serving at Gallagher’s request as a director, officer, employee or agent of another enterprise, against all costs actually and reasonably incurred by him or her in connection with such action, suit or proceeding if he or she acted in good faith and in a manner he or she reasonably believed to be in or not opposed to Gallagher’s best interests and, with respect to any criminal action or proceeding, had no reasonable cause to believe his or her conduct was unlawful. Similar indemnity is permitted to be provided to such persons in connection with an action or suit by Gallagher or in Gallagher’s right, and provided further that such person shall not have been adjudged liable to Gallagher, unless, in view of all the circumstances of the case, the court in which the action or suit was brought determines that such person despite the adjudication of liability is fairly and reasonably entitled to indemnity for such expenses.

Article Twelve of Gallagher’s Amended and Restated Certificate of Incorporation eliminates the liability of Gallagher’s directors for monetary damages for breach of fiduciary duty as a director except where a director breaches his or her duty of loyalty to Gallagher or its stockholders, fails to act in good faith or engages in intentional misconduct or a knowing violation of law, authorizes the payment of a dividend or stock repurchase that is illegal under Section 174 of the DGCL, or obtains an improper personal benefit.

Gallagher also maintains and pays premiums on a directors’ and officers’ liability insurance policy and has entered into indemnity agreements with its directors and officers. The provisions of each indemnity agreement alter or clarify the statutory indemnification in the following respects: (1) indemnity will be explicitly provided for settlements in derivative actions; (2) prompt payment of litigation expenses will be provided in advance of indemnification; (3) prompt indemnification of advances of expenses will be provided unless a determination is made that the director or officer has not met the required standard; (4) the director or officer will be permitted to petition a court to determine whether his or her actions meet the standards required; and (5) partial indemnification will be permitted in the event that the director or officer is not entitled to full indemnification. In addition, each indemnity agreement specifically includes indemnification with respect to actions, suits or proceedings brought under and/or predicated upon the Securities Act and/or the Securities Exchange Act.

The preceding summary is qualified in its entirety by Gallagher’s Amended and Restated Certificate of Incorporation and Amended and Restated By-laws, and the indemnity agreements described above.

Item 21. Exhibits and Financial Statement Schedules.

(a) See Exhibit Index attached hereto and incorporated herein by reference.

(b) Schedules for which provision is made in the applicable accounting regulations of the SEC are either not required under the related instructions, are inapplicable or not material, or the information called for thereby is otherwise included in the financial statements incorporated by reference and therefore has been omitted.

Item 22. Undertakings.

(a) The undersigned registrant hereby undertakes:

(1)(a) To file, during any period in which offers or sales are being made, a post-effective amendment to this registration statement:

(i)(1) To include any prospectus required by section 10(a)(3) of the Securities Act;

(ii)(2) To reflect in the prospectus any facts or events arising after the effective date of the registration statement (or the most recent post-effective amendment thereof) which, individually or in the aggregate, represent a fundamental change in the information set forth in the registration statement. Notwithstanding the foregoing, any increase or decrease in the volume of securities offered (if

the total dollar value of securities offered would not exceed that which was registered) and any deviation from the low or high end of the estimated maximum offering range may be reflected in the form of prospectus filed with the Commission pursuant to Rule 424(b) if, in the aggregate, the changes in volume and price represent no more than 20 percent change in the maximum aggregate offering price set forth in the “Calculation of Registration Fee” table in the effective registration statement; and

(iii)(3) To include any material information with respect to the plan of distribution not previously disclosed in the registration statement or any material change to such information in the registration statement.statement;

provided, however, that paragraphs (1), (2) and (3) above do not apply if the information required to be included in a post-effective amendment by those paragraphs is contained in reports filed with or furnished to the Commission by the registrant pursuant to Section 13 or Section 15(d) of the Securities Exchange Act of 1934 that are incorporated by reference in the registration statement, or is contained in a form of prospectus filed pursuant to Rule 424(b) that is part of the registration statement.

(b) That, for the purpose of determining any liability under the Securities Act, each such post-effective amendment shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

(3)(c) To remove from registration by means of a post-effective amendment any of the securities being registered which remain unsold at the termination of the offering.

(4)(d) That, for the purpose of determining liability under the Securities Act to any purchaser, eachpurchaser:

(1) Each prospectus filed by the registrant pursuant to Rule 424(b)(3) shall be deemed to be part of the registration statement as of the date the filed prospectus was deemed part of and included in the registration statement; and

(2) Each prospectus required to be filed pursuant to Rule 424(b)(2), (b)(5) or (b)(7) as part of a registration statement in reliance on Rule 430B relating to an offering other than registration statements relying onmade pursuant to Rule 430B415(a)(1)(i), (vii) or other than prospectuses filed in reliance on Rule 430A,(x) for the purpose of providing the information required by Section 10(a) of the Securities Act of 1933 shall be deemed to be part of and included in the registration statement as of the earlier of the date itsuch form of prospectus is first used after effectiveness.effectiveness or the date of the first contract of sale of securities in the offering described in the prospectus. As provided in Rule 430B, for liability purposes of the issuer and any person that is at that date an underwriter, such date shall be deemed to be a new effective date of the registration statement relating to the securities in the registration statement to which that prospectus relates, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof. Provided, however, that no statement made in a registration statement or prospectus that is part of the registration statement or made in a document incorporated or deemed incorporated by reference into the registration statement or prospectus that is part of the registration statement will, as to a purchaser with a time of contract of sale prior to such first use,effective date, supersede or modify any statement that was made in the registration statement or prospectus that was part of the registration statement or made in any such document immediately prior to such date of first use.effective date.

(5)(e) That, for the purpose of determining liability of the registrant under the Securities Act to any purchaser in the initial distribution of the securities, the undersigned registrant undertakes that in a primary offering of securities of the undersigned registrant pursuant to this registration statement, regardless of the underwriting method used to sell the securities to the purchaser, if the securities are offered or sold to such purchaser by means of any of the following communications, the undersigned registrant will be a seller to the purchaser and will be considered to offer or sell such securities to such purchaser:

(i)(1) Any preliminary prospectus or prospectus of the undersigned registrant relating to the offering required to be filed pursuant to Rule 424;

(ii)(2) Any free writing prospectus relating to the offering prepared by or on behalf of the undersigned registrant or used or referred to by the undersigned registrant;

(iii)

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(3) The portion of any other free writing prospectus relating to the offering containing material information about the undersigned registrant or its securities provided by or on behalf of the undersigned registrant; and

(iv)(4) Any other communication that is an offer in the offering made by the undersigned registrant to the purchaser.

(b)(f) The undersigned registrant hereby undertakes that, for purposes of determining any liability under the Securities Act, each filing of the registrant’s annual report pursuant to Section 13(a) or Section 15(d) of the Exchange Act (and, where applicable, each filing of an employee benefit plan’s annual report pursuant to Section 15(d) of the Exchange Act) that is incorporated by reference in the registration statement shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

(c) The undersigned registrant hereby undertakes to deliver or cause to be delivered with the prospectus, to each person to whom the prospectus is sent or given, the latest annual report to security holders that is

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incorporated by reference in the prospectus and furnished pursuant to and meeting the requirements ofRule 14a-3 orRule 14c-3 under the Exchange Act; and, where interim financial information required to be presented by Article 3 ofRegulation S-X are not set forth in the prospectus, to deliver, or cause to be delivered to each person to whom the prospectus is sent or given, the latest quarterly report that is specifically incorporated by reference in the prospectus to provide such interim financial information.

(d) The undersigned registrant hereby undertakes as follows:

(1) That prior to any public reoffering of the securities registered hereunder through use of a prospectus which is a part of this registration statement, by any person or party who is deemed to be an underwriter within the meaning of Rule 145(c), the issuer undertakes that such reoffering prospectus will contain the information called for by the applicable registration form with respect to reofferings by persons who may be deemed underwriters, in addition to the information called for by the other items of the applicable form; and

(2) That every prospectus: (i) that is filed pursuant to paragraph (d)(1) immediately preceding, or (ii) that purports to meet the requirements of Section 10(a)(3) of the Securities Act and is used in connection with an offering of securities subject to Rule 415, will be filed as a part of an amendment to the registration statement and will not be used until such amendment is effective, and that, for purposes of determining any liability under the Securities Act, each such post-effective amendment shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

(e)(g) Insofar as indemnification for liabilities arising under the Securities Act may be permitted to directors, officers and controlling persons of the registrant pursuant to the foregoing provisions, or otherwise, the registrant has been advised that in the opinion of the Securities and Exchange Commission such indemnification is against public policy as expressed in the Securities Act and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by the registrant of expenses incurred or paid by a director, officer or controlling person of the registrant in the successful defense of any action, suit or proceeding) is asserted by such director, officer or controlling person in connection with the securities being registered, the registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question whether such indemnification by it is against public policy as expressed in the Securities Act and will be governed by the final adjudication of such issue.

(f)(h) The undersigned registrant hereby undertakes to respond to requests for information that is incorporated by reference into the prospectus pursuant to Items 4, 10(b), 11, or 13 of this Form, within one business day of receipt of such request, and to send the incorporated documents by first class mail or other equally prompt means. This includes information contained in documents filed subsequent to the effective date of the registration statement through the date of responding to the request.

(g)(i) The undersigned registrant hereby undertakes to supply by means of a post-effective amendment all information concerning a transaction, and the company being acquired involved therein, that was not the subject of and included in the registration statement when it became effective.

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SIGNATURES

Pursuant to the requirements of the Securities Act, the registrant has duly caused this registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Itasca,Rolling Meadows, State of Illinois, on the 15th day of November, 2016.2022.

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POWER OF ATTORNEY

We, the undersigned directors and officers, do hereby constitute and appoint Walter D. Bay and Douglas K. Howell, and each of them severally, our true and lawful attorneys-in-fact and agents with full power of substitution and re-substitution for us and in our name, place, and stead, to do any and all acts and things in our name and on our behalf in our capacities as directors and officers and to execute any and all instruments for us and in our names in the capacities indicated below, which said attorneys-in-fact and agents may deem necessary or advisable to enable Arthur J. Gallagher & Co. to comply with the Securities Act of 1933, as amended (the “Securities Act”), and any rules, regulations and requirements of the Securities and Exchange Commission, in connection with this registration statement onForm S-4, including specifically, but without limitation, power and authority to sign for us or any of us, in our names in the capacities indicated below, any and all amendments (including pre- and post-effective amendments) hereto and any related registration statement and amendments thereto and to file the same, with all exhibits thereto, and other documents in connection therewith; and we do each hereby ratify and confirm all that said attorneys-in-fact and agents shall do or cause to be done by virtue hereof.

Pursuant to the requirements of the Securities Act, this registration statement has been signed by the following persons in the capacities and on the dates indicated.

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