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

Date of Report (Date of earliest event reported): February 26, 2013

THE HANOVER INSURANCE GROUP, INC.

(Exact name of registrant as specified in its charter)

440 Lincoln Street, Worcester, Massachusetts 01653

(Address of principal executive offices)

(Zip Code)

Registrant’s telephone number, including area code: (508) 855-1000

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:

Item 5.02. Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; Compensatory Arrangements of Certain Officers.

Compensatory Arrangements of Certain Officers

On February 26, 2013, the following actions were taken with respect to the compensation of the Company’s Chief Executive Officer (“CEO”) and other “named executive officers” (as that term is defined in Item 402 of Regulation S-K) of the Company.

Approval of 2012 Short-Term Incentive Compensation Program Awards

The 2012 short-term incentive compensation program awards for the CEO and other named executive officers (“NEOs”) were approved. The following table lists the awards for our CEO and the NEOs identified below:

2010 Performance-Based Restricted Stock Units

Based upon the Company’s level of average net written premium growth and property and casualty return on equity achieved during the performance period (2010-2012) (calculated in accordance with plan rules to exclude the impact of certain catastrophe losses), the 2010 performance-based restricted stock units (“PBRSUs”) would have permitted a payout at approximately 70% of the target level award. However, in light of the Company’s financial performance, including catastrophe losses over the period, the Compensation Committee determined that there should be no payments under the program; accordingly, awards granted to Mr. Eppinger, Ms. Zuraitis and Mr. Huber, with reported target values on the grant date of $590,100, $273,975 and $110,644, respectively, were forfeited in their entirety.

Approval of the 2013 Short-Term Incentive Compensation Programs for Named Executive Officers

Short-term incentive compensation plans were approved as follows: (1) the 2013 Executive Short-Term Incentive Compensation Program (the “2013 Executive STIP”) for our NEOs, other than Mr. Stuchbery; and (2) the 2013 Chaucer Annual Bonus Plan (the “2013 Chaucer STIP”) for Mr. Stuchbery.

(1) The 2013 Executive STIP was established pursuant to the Company’s shareholder approved 2009 Short-Term Incentive Compensation Plan (filed as Exhibit 10.15 to the Company’s Annual Report on Form 10-K). Individual awards for the NEOs provide for target awards ranging from 75% to 120% of base salary. The actual amount of the award, however, may range from zero to a maximum of 200% of target, based on the Company achieving certain levels of adjusted segment income. The actual amount of each executive officer’s award, however, is dependent on the level of achievement of the Company’s performance targets, such executive officer’s individual performance and such other factors as the Compensation Committee may determine, but in no event may any such award exceed the amount determined in accordance with the pre-established adjusted segment income performance metric. For 2013, awards, if any, are payable in the first fiscal quarter of 2014.

(2) The 2013 Chaucer STIP was established to provide cash compensation opportunities for certain Chaucer employees, including Mr. Stuchbery. Mr. Stuchbery’s target award under the program was fixed at 100% of his base salary. The actual amount of the award, however, ranges from zero to a maximum of 200% of target, based on Chaucer’s 2013 post-tax return on equity (a return on equity calculation obtained by dividing Chaucer’s 2013 after-tax segment income by Chaucer’s average consolidated equity, as adjusted). Once the overall plan funding level has been determined, each participant’s actual payment under the plan is determined based according to the following formula: 50% of the funding pool is distributed pro rata to participants based upon their individual target awards and the remaining 50% of the funding pool is distributed to participants based upon individual performance. Each participant’s total award, if any, is paid in two equal tranches: the first to be paid in March 2014 and, provided the participant remains employed by Chaucer through such date, the second tranche is to be paid in January 2015. Notwithstanding the foregoing, the Compensation Committee retains the discretion to increase or decrease the funding pool and Mr. Stuchbery’s individual award based upon any factor it deems appropriate.

Approval of the 2013 Long-Term Incentive Program

The 2013 Long-Term Incentive Program (the “2013 LTIP”) for the CEO and other executive officers, including the Company’s other NEOs, was approved. The 2013 LTIP was established pursuant to the Company’s 2006 Long-Term

Incentive Plan (filed as Exhibit 10.8 to the Company’s Annual Report on Form 10-K) (the “2006 Plan”). The 2013 LTIP provides for a combination of awards of performance-based restricted stock units (“PBRSUs”), stock options (“Options”) and time-based restricted stock units (“RSUs”).

Except with respect to Mr. Stuchbery, the PBRSUs vest on the third anniversary of the date of grant, but only to the extent the Company’s three-year (2013-2015) total shareholder return as compared to a pre-established peer group (relative total shareholder return or “RTSR”) places the Company’s performance above a certain percentile. Participants must be employees of the Company as of the vesting dates for the PBRSUs to vest, except as otherwise provided with regard to death, disability or change-in-control. The actual PBRSU award may be as low as zero, and as high as 150% of the target award, based on the RTSR actually achieved.

Unlike the other NEOs, Mr. Stuchbery’s PBRSU award was divided equally into two components. One-half of Mr. Stuchbery’s PBRSU award is on the same terms as described above, except for his award, RTSR is measured over a two-year period (2013-2014) and vests on the second anniversary of the date of grant. The second half of Mr. Stuchbery’s PBRSU award is on the following terms:

Except with respect to Mr. Stuchbery, whose RSUs vest on the second anniversary of the grant date, the RSUs vest on the third anniversary of the date of grant. Participants must be employees of the Company as of the vesting dates for the RSUs to vest, except as otherwise provided with regard to death, disability or change-in-control.

The Options vest as to one-third of the shares on each of the first three anniversaries of the grant date. Participants must be employees of the Company as of the vesting dates for the Options to vest, except as otherwise provided with regard to disability or change-in-control. Each Option has a ten year term and an exercise price of $42.49 per share, which was the closing price per share of THG’s common stock as reported on the New York Stock Exchange on the date of grant (February 26, 2013).

The following table sets forth the number of PBRSUs (at target), RSUs and Options granted to the following executive officers.

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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.

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