"Jack has the distinction of having worked and succeeded in some of the toughest jobs in Washington and the private sector," Obama said Thursday.

During his 2010 confirmation hearing, Lew sought to distance himself from being seen as a bank executive with a role in the real estate bubble and bust. He pointed out that he had been a manager and not an investment adviser. In an internal memo announcing Lew's appointment, Citi said he would be responsible for operations, technology, human resources, legal, finance and regional coordination.

Despite his tenure at Citigroup, Wall Street is suspicious of Lew's lack of "significant experience in financial regulatory matters and the financial markets," said Washington analyst Brian Gardner with Keefe, Bruyette & Woods.

"While he can undoubtedly learn the material on the job, we question whether he has sufficient relationships with the banking industry in the U.S. and abroad, which can be critical during a financial crisis," Gardner said in research note.

Lew also worked as a financial administrator at New York University from 2001 to 2006. He received an undergraduate degree at Harvard University and a law degree from Georgetown University.

Lew has developed a reputation as a details guy, who regularly logs in 16-hour days at the White House. He observes the Jewish Sabbath of leaving work before sundown on Friday and abstaining from work on Saturdays. However, he is known to have worked Saturdays in urgent situations.

One detail about Lew that came to light this week is his loopy, illegible signature, which will grace the lower right-hand side of each new dollar bill if he's confirmed as Treasury Secretary. Obama joked about his nominee's penmanship at Thursday's press conference, saying Lew "assures me that he is going to work to make at least one letter legible in order not to debase our currency."

CNNMoney's James O'Toole contributed reporting.