Over his 30-year career, John A. Thain has risen to top executive positions at Goldman Sachs, the New York Stock Exchange and Merrill Lynch.

On Monday, he will start over at a humbler institution: the CIT Group, the lender to small and midsize businesses that emerged two months ago from a swift bankruptcy.

In doing so, Mr. Thain, 54, is seeking to leave behind the controversies that haunted his final days at Merrill after it was acquired by Bank of America, a deal he helped engineer to save the brokerage during the height of the financial crisis.

As CIT’s new chairman and chief executive, he will try to rebuild a company whose bet on increasingly risky businesses led it to the brink of dissolution, only to be saved at the last minute by its creditors.