Material Event - Form 8-K

On February 12, 2024, Arthur J. Gallagher & Co. (the "Company") entered into an underwriting agreement (the "Underwriting Agreement") with BofA Securities, Inc. and J.P. Morgan Securities LLC as representatives of the underwriters named in Schedule A thereto (the "Underwriters"), with respect to the offering and sale of $500 million aggregate principal amount of its 5.450% Senior Notes due 2034 (the "2034 Notes") and $500 million aggregate principal amount of its 5.750% Senior Notes due 2054 ("2054 Notes," and together with the 2034 Notes, the "Notes"). The Underwriting Agreement is filed herewith as Exhibit 1.1.

The Notes offering has been registered under the Securities Act of 1933, as amended, pursuant to a shelf registration statement on Form S-3,file no. 333-277002(the "Registration Statement"), filed with the U.S. Securities and Exchange Commission (the "SEC"). The Notes will be issued pursuant to an indenture, dated as of May 20, 2021 (the "Indenture"), between the Company and The Bank of New York Mellon Trust Company, N.A., as trustee (the "Trustee"), as modified and supplemented in respect of the Notes by an Officers' Certificate pursuant to the Indenture, dated as of February 15, 2024 (the "Officers' Certificate"). The relevant terms of the Notes, the Indenture and the Officers' Certificate are further described under the caption "Description of Notes" in the prospectus supplement dated February 12, 2024, filed with the SEC by the Company on February 14, 2024. This description is incorporated in this Item 8.01 by reference. The Notes offering closed on February 15, 2024.

The Officers' Certificate (including the forms of the Notes) is filed as an exhibit to this Current Report on Form 8-Kand incorporated herein by reference. The description of the Officers' Certificate (including the forms of the Notes) in this Current Report on Form 8-Kis a summary and is qualified in its entirety by the terms of the Officers' Certificate (including the form of the Notes). The Company is filing this Current Report on Form 8-Kin order to file with the SEC certain items that are to be incorporated by reference into the Registration Statement. The opinion of Gibson, Dunn & Crutcher LLP relating to the validity of the Notes is filed herewith as Exhibit 5.1.