This time, however, the selection of a Goldman insider is at odds with statements made by Mr. Trump during the presidential campaign. He repeatedly attacked the financial elite — and Goldman Sachs in particular.
In a commercial that ran in the closing days of the campaign, Mr. Trump warned about "a global power structure that is responsible for the economic decisions that have robbed our working class, stripped our country of its wealth and put that money into the pockets of a handful of large corporations and political entities." The face of Goldman's chief executive, Lloyd C. Blankfein — Mr. Cohn's longtime friend and collaborator — was among the images that flashed ominously on the screen.
And Mr. Trump criticized both Hillary Clinton and a primary opponent, Senator Ted Cruz, Republican of Texas, over their ties to the investment bank. "I know the guys at Goldman Sachs," Mr. Trump said at one primary debate. "They have total, total control" over Mr. Cruz, he said. "Just like they have total control over Hillary Clinton."
Since his election, however, Mr. Trump has stocked his future cabinet with a number of Goldman alumni, including Mr. Mnuchin, a hedge fund manager and a former Goldman trader, and Stephen K. Bannon, a former Goldman banker who is now Mr. Trump's chief strategist.
Unlike those two, Mr. Cohn is a longtime top Goldman executive who was helping to guide the firm before and during the financial crisis.
In Mr. Cohn, Mr. Trump is not only turning to yet another Goldman hand — and a registered Democrat — but he is also choosing a financier whose thinking about the economy has stood in contrast to the president-elect's more nationalistic views.
At a conference in Florida soon after the election, Mr. Cohn said the big problem facing the country and the world was a "global growth issue."
"We're trying to solve it with domestic policy," he said. "It's not going to work."
While Mr. Trump has criticized companies that have moved their work force overseas, Mr. Cohn has been candid about Goldman's international outlook: "We have a globalized work force, so when I need to go out and hire the incremental worker, I go out and look around the world and see where that incremental worker is available."
Mr. Cohn, though, has agreed with Mr. Trump about the need to lighten the regulations that have been imposed on banks like Goldman since the financial crisis.
In another interview at the conference, with CNBC, Mr. Cohn said he was "cautiously optimistic" about a Trump administration.
"We're all giving President-elect Trump and his transition team the benefit of the doubt," Mr. Cohn said. "We're all waiting to see what happens."
Mr. Cohn, 56, rose through the ranks of Goldman as a trader and is known for his gruff, no-nonsense demeanor, which could help explain his rapport with the plain-spoken Mr. Trump.
In recent years, Mr. Cohn has been in line to take over the firm from Mr. Blankfein, and his departure will open the door to a new crop of candidates looking to lead the firm.
While he has been registered as a Democrat, Mr. Cohn has donated to both political parties. He has given tens of thousands of dollars to Democrats and Democratic campaign committees.
Yet more important, he has become friends with Jared Kushner, Mr. Trump's son-in-law and close adviser.
The decision follows an extended courtship in which the Goldman executive visited with Mr. Trump three times, most recently on Thursday.
Leaving Wall Street to take a top government post could provide a huge financial gain for Mr. Cohn.
He would probably have to sell his Goldman holdings to avoid conflicts of interest with his new role, which would normally generate a big tax bill immediately. But tax regulations allow executive branch appointees to roll the proceeds of such a sale into Treasury securities and defer capital gains taxes.
Mr. Cohn owned 872,712 shares in Goldman as of Nov. 14, according to Standard & Poor's Global Market Intelligence. As of Friday afternoon's stock price, that stake was worth about $209 million.
Goldman Sachs has already been a beneficiary of the coming Trump administration. Mr. Trump has promised to push back on financial regulations passed since the financial crisis, which have come down particularly hard on Goldman. Since the election, shares of banks and other financial institutions have risen sharply; Goldman's is up 34 percent.
On Friday, Goldman's shares edged up slightly.
Mr. Cohn grew up in the suburbs of Cleveland, the son of a real estate developer and electrical contractor. He later attended Gilmour Academy, a private school in the area, and then American University, though he has often spoken of his struggles with dyslexia.
After a brief stint at U.S. Steel in his home state — to appease his father, Mr. Cohn said in a speech — he turned to the New York Mercantile Exchange in 1983, where he eventually turned to trading silver. He was recruited to Goldman seven years later and became a fast-rising star at the investment bank, following the ascent of his friend Mr. Blankfein.
In 1994, Mr. Cohn joined the vaunted partnership at Goldman, in a class that included a number of financial luminaries: Eric M. Mindich, now a hedge fund mogul; J. Michael Evans, formerly a top Goldman executive in China who is now at the Alibaba Group; and Mr. Mnuchin himself.
By 2006, Mr. Cohn had become co-president and co-chief operating officer of the firm when Mr. Blankfein took the helm as chief executive. He took sole ownership of the No. 2 spot in 2009 and solidified his role as the heir apparent.
Over his career, the 6-foot-3 Mr. Cohn has been known for a brusque and intimidating presence, reportedly looming over traders at their desks. But he has softened that approach over the years as he became more of a financial diplomat, flying to Washington, the World Economic Forum in Davos, Switzerland, and other centers of power.