On Capitol Hill, Mr. Muzinich is often paired with Mr. Cohn’s top deputy, Shahira Knight, to convey the administration’s views in big tax meetings. He speaks almost daily with Mr. Mnuchin and, according to congressional aides, it is apparent that he has the authority to speak on behalf of the secretary.

“Justin’s leadership and expertise has been essential in advancing tax reform, which has been our administration’s top domestic priority,” Mr. Mnuchin said in a statement.

Mr. Muzinich, who holds an M.B.A. from Harvard and a law degree from Yale, was relatively unknown in tax policy circles before he was plucked from Wall Street to join the Treasury Department. His most notable prior experience in the world of policy came while working on the campaign of one of Mr. Trump’s arch rivals: Jeb Bush.

Mr. Bush first learned of Mr. Muzinich because of his work as an informal adviser to the 2012 presidential campaign of Mitt Romney, another nemesis of Mr. Trump. While Mr. Trump’s personality-driven campaign was lean on policy, Mr. Muzinich was busy overseeing a team that cranked out detailed proposals for Mr. Bush on taxes, education and health care. Those attracted little public attention for Mr. Bush’s failed White House bid, but the experience gave Mr. Muzinich a taste of doing thankless work in a political caldron and managing big personalities under pressure.

One of the earliest iterations of the tax framework unveiled by Mr. Trump and congressional Republicans last month was the one that Mr. Muzinich drafted for Mr. Bush in 2015. Like the current plan, that one would have collapsed the personal income tax brackets from seven to three and lowered the rates. It would have doubled the standard deduction, reduced the corporate tax rate to 20 percent from 35 percent, and switched the United States to a “territorial” tax system, as the current plan would also do.

“What impressed me beyond his business experience, which is pretty unique, is that he’s just a policy wonk,” Mr. Bush said in an interview. “Justin has a desire to serve and he’s getting to do it at a high level, and I’m really proud of him for doing it.”

Policy has long been a passion for Mr. Muzinich, who regularly sought out opportunities while on Wall Street to publish essays on subjects ranging from foreign aid to the mandate of the Federal Reserve. Some of his earliest writing on tax policy could be considered heretical in an administration with a guiding principle of “America First.” In a 2007 Op-Ed article in The New York Times, Mr. Muzinich and a co-author, Eric Werker, called on Congress to offer tax credits to companies that build factories in developing countries and to offset the lost revenue with reductions in foreign aid.