UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

Form 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of

the Securities Exchange Act of 1934

June 1, 2015

Date of Report: (Date of earliest event reported)

ARTHUR J. GALLAGHER & CO.

(Exact name of registrant as specified in its charter)

Two Pierce Place, Itasca, Illinois 60143-3141, (630) 773-3800

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

Not Applicable

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Arthur J. Gallagher & Co. (“Gallagher”) held its Annual Meeting of Stockholders on June 1, 2015 (the “Annual Meeting”). At the Annual Meeting, five items were submitted to Gallagher’s stockholders. The items are described in more detail in Gallagher’s definitive proxy statement filed with the Securities and Exchange Commission on April 16, 2015. The final voting results are set forth below:

Item 1: Election of nine directors, each to serve for a one-year term until Gallagher’s 2016 Annual Meeting of Stockholders:

* Broker non-votes represent shares held by broker nominees for beneficial owners that were not voted with respect to a non-routine proposal because the broker nominee did not receive voting instructions from the beneficial owner and lacked discretionary authority to vote the shares. If a broker does not receive voting instructions from the beneficial owner, a broker may vote on routine matters, such as the ratification of the Independent Registered Public Accounting Firm, but may not vote on non-routine matters. Broker non-votes are counted for the purpose of determining the presence of a quorum but are not counted for the purpose of determining the number of shares entitled to vote on non-routine matters such as the election of Directors, approval of the Gallagher Employee Stock Purchase Plan, approval of the Gallagher Senior Management Incentive Plan and the advisory vote on the compensation of Gallagher’s named executive officers.

The slides containing information presented at the Annual Meeting are furnished as Exhibit 99 to this report. This exhibit may also be accessed at Gallagher’s website (www.ajg.com) under the heading Investor Relations.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.