Over the last few years, Mr. Mnuchin has been having trouble walking. This year, he skipped Art Basel in Switzerland for the first time in about two decades. So people were really happy to see him back out on Wednesday, playing a role in what was arguably the most exciting sale of the week.

There was not likely a big payday for the services he rendered Wednesday night, several art world figures said. But had Mr. Heller sat in the gallery doing the bidding, it would immediately have drawn attention to Mr. Cohen. And Mr. Mnuchin likes being on the floor in the middle of the action.

He finds moments like this a parallel to his previous career in trading.

“It was an intense business,” he said of working at Goldman Sachs. “It was very competitive and I was successful at building relationships with serious institutional people, that I could work for them and myself at the same time. That I could serve as their agent and be their principal. And I think I’ve carried that over somewhat into this.”