One prominent woman in Obama's Cabinet, Labor Secretary Hilda Solis, resigned her post Wednesday. No successor has been named.
Lew was a top aide in the 1980s to House Speaker Tip O'Neill, a Massachusetts Democrat, playing a role in the Social Security deal between the speaker and President Ronald Reagan in 1983.
Before becoming Obama's chief of staff, Lew was director of the Office of Management and Budget, a post he also held back in the Clinton administration, serving from 1998 to early 2001. While running OMB during the Clinton administration, Lew helped negotiate a balanced budget agreement with Congress, something that has eluded Washington ever since.
When Obama named him chief of staff one year ago, the president praised him as the "only budget director in history to preside over budget surpluses for three consecutive years."
"His resume is tailor-made for what is most important right now," said Diane Swonk, chief economist at Mesirow Financial in Chicago. On Wall Street, Lew was managing director and chief operating officer of Citi Global Wealth Management and then Citi Alternative Investments. At the start of the Obama administration, he oversaw international economic issues at the State Department.
Despite his stint on Wall Street, Lew doesn't have the type of financial experience that Geithner brought to the job at the height of the financial crisis in 2009. Indeed, there's not as much of a premium on those skills now as the nation's attention has turned from bank bailouts to fiscal confrontations and brinkmanship.
Still, Lew will have to address other significant challenges, including completing implementation of the financial regulatory overhaul of 2010.
Lew will probably play a key role in deciding the fate of government-controlled Fannie Mae and Freddie Mac, the federal housing agencies partly blamed for the collapse of the housing market.
Internationally, he will also be the administration's point man on issues related to China's integration into the global economy and Europe's sovereign debt and financial struggles. The issues aren't foreign to Lew. While a deputy secretary of state early in the Obama administration, Lew managed the State Department's international economic policy portfolio.
For all that, Lew faces a better landscape than Geithner did when he stepped into the post at the start of Obama's first term.
"The basic financial position and economic position of the country is much stronger today that it was four years ago," said Michael Barr, who was assistant treasury secretary for financial institutions in 2009 and 2010. "That's a significant advantage for a treasury secretary coming in."