When a group of protesters gathered outside his Los Angeles house in October 2011 to voice their anger at OneWest’s foreclosure practices, police officers were called to disperse the crowd. He responded later by periodically scrubbing the internet of any reference to his home address to protect the privacy of his three young children, according to a filing Mr. Mnuchin made in a 2014 divorce proceeding.

He is more comfortable in his own element — wealthy hedge fund managers like himself.

Days after being tapped by Mr. Trump to be his finance chairman in May, Mr. Mnuchin flew out to a hedge fund conference at the Bellagio hotel in Las Vegas. There, in a swirl of rock concerts, pool parties and casino nights, he schmoozed with billionaires.

Mr. Mnuchin dined with Kenneth C. Griffin, the Chicago hedge fund titan, and government veterans like David H. Petraeus, the former C.I.A. director and retired general, and John A. Boehner, the former House speaker. He shook hands with Leon Cooperman, a 73-year-old pioneer in the industry and another Goldman Sachs alumnus.

The courting paid off: Not long after the hedge fund conference, Mr. Mnuchin helped bring the billionaire investor John A. Paulson on as a donor, as well as Wilbur L. Ross Jr., now Mr. Trump’s commerce secretary nominee.

It is these ties, however, that are expected to be fertile ground of investigation for senators who must confirm his selection as Treasury secretary.

“I think he would be controversial — former Treasury secretaries have been top bankers and businessmen or lawyers and they were well-recognized top leaders in their professions,” said Prof. Richard Sylla at the Stern School of Business of New York University, whose research focuses on financial history and economics. “But Mnuchin wasn’t that famous as a Goldman Sachs guy, and probably not even as a hedge fund guy, so why is he there? Well, he took the job of raising money for Trump.”

Mr. Mnuchin shares a network with some of the biggest names in the $3 trillion hedge fund industry, but his work with Mr. Paulson is of particular interest.