Since being pulled back from the brink in the 2008 financial crisis, American International Group has shrunk significantly. And activist investors have further pushed to split the insurer.

The company’s new chief executive, however, pledged on Monday to focus on growth.

“Let me be clear, I am here to grow A.I.G.,” the executive, Brian Duperreault, said at the company’s consumer insurance investor day on Monday.

“I recognize the value of the company’s multiline structure,” he said. “I didn’t come here to break the company up. I came here to grow it.”

His appointment is a homecoming. Mr. Duperreault, 70, worked 21 years at A.I.G. before leaving in 1994. He was a lieutenant to Maurice R. Greenberg, who built the New York-based company into a global colossus before resigning as chief executive amid accounting investigations in 2005.

Mr. Duperreault replaces Peter D. Hancock, who abruptly announced plans to resign as A.I.G.’s chief executive in March after shareholders lost faith in a two-and-a-half-year turnaround effort.

Mr. Hancock, a former J.P. Morgan executive, had come under pressure from activist investors. In 2015, the billionaire Carl C. Icahn publicly called for A.I.G. to be split up and to get new leadership.

On Monday, Mr. Icahn, who has a nearly 5 percent stake in A.I.G., applauded the new chief executive.

“Very pleased the $AIG board is finally making some of the much-needed changes we’ve been advocating the last 18 months,” he wrote on Twitter.

Shares of A.I.G. were up nearly 0.9 percent in afternoon trading on Monday.

Mr. Duperreault, who founded and was chief executive of the Hamilton Insurance Group of Bermuda, does not come cheap. He will receive an annual salary of $1.6 million; a short-term annual incentive bonus of $3.2 million, which would be prorated for 2017; and a long-term incentive award of $11.2 million.

He also would receive a one-time cash award of $12 million as compensation for unvested equity that he forfeited by leaving Hamilton and the option to purchase 1.5 million shares of A.I.G. stock as a one-time, sign-on award.

Mr. Duperreault becomes the sixth chief executive to run the insurer since the departure of Mr. Greenberg, who is known as Hank.

In joining A.I.G., Mr. Duperreault will find a company much different from when he left and will face a difficult task in bringing the insurer back to its pre-crisis heights.

A.I.G. was once considered the gold standard for insurance companies as it expanded through acquisitions engineered by Mr. Greenberg, its longtime leader.

The insurer nearly collapsed in September 2008 and received a $185 billion government bailout. In recent years, A.I.G.’s performance has lagged its peers, despite efforts by its leadership to simplify the company and trim costs.

Investors have been frustrated by the slow pace of recovery at A.I.G., but they were particularly rattled in February by the quarterly loss of $3.04 billion, which was larger than expected.

The loss was largely a result of a $5.6 billion increase in reserves to cover potential claims. Shares of A.I.G. tumbled 9 percent the day after the results were announced.

Mr. Duperreault started at A.I.G. in 1973 and rose through its executive ranks, leaving the company in 1994 for the top job at ACE.

He served as chief executive of ACE until 2004, when he was replaced by Evan G. Greenberg, Mr. Greenberg’s son and now the chief executive of Chubb. Mr. Duperreault served another two years as ACE’s nonexecutive chairman until he retired in 2006.

Mr. Duperreault returned to the insurance business two years later as president and chief executive of the professional services firm Marsh & McLennan Companies.

He retired again in 2012, only to be drawn back to the industry again in 2013 when he began running Hamilton, which he helped found.

“Brian is uniquely qualified to lead A.I.G. at this important time. Brian has spent his entire career in insurance,” Douglas M. Steenland, the company’s chairman, said in a news release.

“He is a hands-on leader who has consistently delivered strong bottom-line results,” Mr. Steenland added.

A.I.G. separately announced that it had agreed in principle to acquire Hamilton’s platform in the United States as part of a push to accelerate its application of data science and analytics to improve its insurance underwriting.