In January, Brian Schreiber was working nearly around the clock preparing an investor presentation on American International Group Inc.’s new direction, called Simplifying AIG. On Feb. 15, Schreiber simplified right out of a job.

Just as AIG prepares to sell billions in assets, the company is losing its biggest dealmaker amid tensions in the executive suite.

With his thick-rimmed glasses and slicked-back hair, Schreiber, 50, could be arrogant, political and disdainful of others, traits that may have prevented him from getting AIG’s top job, according to bankers and colleagues who worked with him. But over his 19 years with AIG, nobody could match his knowledge of the company and the ferocity of his advocacy, they said.

“Each successive CEO knew that Brian knew more about the inner workings of AIG, which is an enormously complex beast, than anybody else — period,” said Christopher Cole, the former co-chairman of investment banking at Goldman Sachs Group Inc.

New Opportunities

In a statement issued through AIG, Schreiber said he was leaving to pursue new opportunities. He declined to comment further. The insurer’s senior executives often disagreed on strategy, according to people with direct knowledge.

“Our entire executive leadership team partnered closely together and were completely aligned on the development of our strategy,” AIG said in an e-mailed statement.

Schreiber’s tenure at AIG spanned the company’s era of dominance and growth as the world’s biggest insurer, its calamitous fall in the subprime-mortgage debacle, its post-bailout return to profitability and its recent tussle with activist investors who want to break it into smaller pieces. He worked for six chief executive officers and arranged transactions worth $265 billion. In 2008, he helped negotiate a government rescue that would swell to $182.3 billion.

Shares Fell

One of the activist investors, the hedge-fund firm run by billionaire John Paulson, considered Schreiber capable of executing the restructuring they favored, people with direct knowledge said in January. AIG subsequently said Paulson would be nominated to its board. A representative for Paulson & Co. declined to comment. AIG shares fell 1.8 percent on Feb. 16, the day after Schreiber left, the sharpest drop in the Standard & Poor’s 500 Insurance Index.

“Brian served with passion and dedication in several key operating and transactional roles,” CEO Peter Hancock said in an e-mail sent to AIG employees after 6 p.m. on Feb. 15, the Presidents Day holiday. “Brian has demonstrated his deep knowledge of the company and industry expertise at every turn, and most recently, was instrumental as chief strategy officer in developing and driving the roll-out of our new strategy.”

That strategy will be implemented with few executives who were around when AIG’s market value surpassed $200 billion. While AIG has shrunk since then, it still operates in more than 100 countries with more than 60,000 employees.

Executive Exodus

Other recent departures from AIG include Seraina Maag, who oversaw regional operations, John Doyle, who led the business serving commercial clients, and Chief Financial Officer David Herzog.

Hancock, who joined the insurer in 2010 after years as a bank executive, moved Schreiber to his strategy role from deputy chief investment officer last year. That cleared room for Hancock’s former J.P. Morgan & Co. colleague, Doug Dachille, to manage the insurer’s portfolio. Alon Neches will take on Schreiber’s duties overseeing mergers and acquisitions and report to the new CFO, according to a person familiar with the change. No one will hold the post of strategy chief. “We have a dynamic new leadership team that is highly skilled and well-positioned to lead our company’s continuing transformation,” AIG said in its statement.

Largest Insurer

Schreiber graduated from New York University and received an MBA from Columbia University. His resume included Lehman Brothers Holdings Inc. and closely held Bass Brothers Enterprises Inc. when AIG’s Maurice “Hank” Greenberg hired him in 1997.

Greenberg built AIG into the world’s largest insurer before departing in 2005 amid a probe into the company’s accounting. He and Schreiber worked on AIG’s most transformative deals, including the 2001 takeover of American General Corp. for more than $20 billion. People who worked with the duo described Schreiber as “Hank’s helper” and “wunderkind.”

Their close relationship crumbled when Schreiber opted to stay at AIG after Greenberg’s exit, according to Morris Offit, an AIG board member from 2005 to 2013. Schreiber worked with regulators to review the company’s books, people with direct knowledge said. AIG eventually restated earnings and settled the issue with regulators including the U.S. Securities and Exchange Commission and the New York Attorney General’s office as part of a $1.64 billion agreement.

“Be careful with him,” Greenberg said about Schreiber. “He’s not trustworthy.” He declined to comment further.

Face of AIG

During the bailout, government officials were frustrated by Schreiber and other senior executives, according to Jim Millstein, the U.S. Treasury Department’s restructuring chief at the time. They felt managers should have known more about AIG’s lack of financial strength, which led to the company’s near-fatal collapse in September 2008, he said. Schreiber won over Treasury officials when he helped them understand the company’s sprawling operations, Millstein said.

“Brian knew where all the bodies were buried,” said Millstein, who now heads his own firm. “He knew who could be trusted and who was a thief.”

Schreiber worked with some of the world’s most prominent bankers, including the late Jimmy Lee of JPMorgan Chase & Co., Blackstone Group LP’s John Studzinski and Ruth Porat, who was at Morgan Stanley. “Brian really has been the face of AIG for Wall Street,” Offit said.

MetLife Deal

During 2010 negotiations on a $15.5 billion agreement to sell a life-insurance division to MetLife Inc., Schreiber surprised Goldman Sachs bankers and MetLife executives by walking out of the room after they made what Schreiber thought were unreasonable demands, people familiar with the talks said. The ploy worked. Schreiber was able to squeeze concessions even though MetLife was the only prospective buyer, helping AIG win an additional $1 billion by demanding partial payment in MetLife shares that subsequently increased in value, a person with direct knowledge said.

“We were very pleased with the price and deal structure, as well as the terms and significant enhancements” that MetLife received in the sale, John Calagna, a spokesman for the buyer, said in an e-mail. Michael DuVally, a Goldman Sachs spokesman, declined to comment.

Schreiber “just grinds you until the result is about the very, very best you think you could ever do,” said Cole, the former Goldman Sachs banker. “You come away from it sweaty and bloody, but my goodness, the result for AIG has been superb.”

Les Paul

One of the enduring images of Schreiber for colleagues was his grace under pressure during the 2008 crisis. Nero might have fiddled as Rome burned; Schreiber played a Gibson Les Paul.

The musically inclined dealmaker strummed the red-and-white electric guitar he kept in his Manhattan office just before AIG accepted government assistance that would eventually be repaid with a $22.7 billion profit for the U.S.

“It was like being on the Titanic, and he was the second officer,” said J. Christopher Flowers, CEO of an eponymous buyout firm that considered participating in a takeover of AIG in 2008. “Brian did a pretty good job under terrible circumstances.”