Federal Reserve Governor Kevin Warsh, who was one of Chairman Ben S. Bernanke’s closest financial-crisis advisers before becoming the only governor to question the expansion of record monetary stimulus in November, resigned after five years at the central bank.

Warsh, 40, a former investment banker who was the youngest- ever Fed governor when then-President George W. Bush appointed him in 2006, will leave “on or around March 31,” he said in a letter today to President Barack Obama that was released by the Fed in Washington. His term would have run through January 2018.

His departure may give Bernanke a stronger hand to complete or potentially expand $600 billion in Treasury purchases through June. At the same time, Bernanke loses a link to Wall Street executives and Republican politicians as he carries out Congress’s overhaul of financial regulation and faces criticism from a political party that in the midterm election gained control of the U.S. House.

“I am honored to have served at a time of great consequence,” Warsh said in his resignation letter. Bernanke said in a statement that Warsh’s “intimate knowledge of financial markets and institutions proved invaluable during the recent crisis.”

Warsh’s resignation opens a second vacancy on the seven-member Board of Governors and leaves Elizabeth Duke, a former community banker, as the only governor not appointed or reappointed by Obama. Duke’s term expires in January 2012, and she can stay after that until a replacement is appointed. Obama’s nomination of Peter Diamond, a Nobel Prize-winning economist from the Massachusetts Institute of Technology, is pending in the Senate again after failing last year.

Anti-Inflation Stance

Warsh staked out an anti-inflation stance on monetary policy in September 2009, when he published a Wall Street Journal op-ed and gave a speech saying the Fed may need to raise interest rates with “greater force” than it has in the past. In June, he said any decision to expand the $2.3 trillion balance sheet must be subject to “strict scrutiny.”

On Nov. 8, he said in an op-ed and speech that the Fed’s Treasury buying “poses nontrivial risks” even after he voted to support the stimulus. He hasn’t publicly discussed his views on the purchases since November and backed the policy at the Fed’s subsequent meetings in December and January.

‘Loosen Policy’

“When non-traditional tools are needed to loosen policy and markets are functioning more or less normally — even with output and employment below trend — the risk-reward ratio for policy action is decidedly less favorable,” Warsh said in the speech in New York. “As a result, we cannot and should not be as aggressive as conventional policy rules — cultivated in more benign environments — might judge appropriate.”

John Ryding, a former Fed researcher who’s now chief economist at RDQ Economics LLC in New York, said that day the speech was a “soft dissent” that expressed concern about the policy without a formal vote against it. Warsh “needs to ‘man up’ and put his vote where his mouth is,” Stephen Stanley, an economist who’s criticized the Fed stimulus, said in a Nov. 8 research note.

Warsh’s ambitions go back to his high school days near Albany, New York, where he had a business buying and distributing neon novelties, according to a 1987 article in the Albany Times-Union.

As a freshman at Stanford University in California, Warsh pestered political-science professor David Brady to attend a senior seminar that wasn’t open to first-year students, Brady said in 2009.

Best Grade

“He was so persistent,” said Brady, who became his thesis adviser at Stanford. “He said he would get the best grade in the class, and he did.”

After graduating from Stanford, Warsh earned a law degree from Harvard University but never practiced, opting instead to join Morgan Stanley in New York, where he worked in the mergers and acquisitions department from 1995 to 2002. He then joined the White House, advising on policies including the government-chartered home-finance companies Fannie Mae and Freddie Mac.

Warsh was an architect of the terms the Treasury dictated to nine of the biggest U.S. banks in October 2008 in return for a $125 billion injection of government funds. He played a central role in negotiating the sale of the ailing Wachovia Corp., mediating a takeover fight that erupted between Citigroup Inc. and Wells Fargo & Co.

Intensified Crisis

Days before Lehman Brothers Holdings Inc.’s bankruptcy in 2008 intensified the crisis, a Fed staff member e-mailed Warsh to say she hoped “we don’t have to protect” some Lehman debt holders, according to documents released by the Financial Crisis Inquiry Commission. Warsh replied an e-mail 30 minutes later that “I hope we dont (sic) protect anything!”

In January 2009, Warsh was passed over for the presidency of the New York Fed in favor of William Dudley, a former Goldman Sachs Group Inc. economist and leading advocate of the Fed’s stimulus that’s been dubbed QE2 by investors for a second round of quantitative easing.

Warsh has served as the Fed’s representative to the Group of 20 and the Board of Governors’ emissary to emerging and advanced economies in Asia. He also managed Fed operations and personnel as the governor assigned to administration.

Warsh in 2002 married Jane Lauder, an heir to her grandmother Estee Lauder’s cosmetics fortune, making him wealthier than the rest of the Fed governors combined. His wife is the global president and general manager of Estee Lauder Cos.’ Origins and Ojon brands and is on the company’s board of directors.