Mnuchin started his career in the early 1980s at Salomon Brothers. He moved to Goldman Sachs in 1985. Mnuchin was one of the instrumental people in the promotion of financial instruments like Collateralized Debt Obligations (CDO) and Credit Default Swaps (CDS).
Mnuchin spent 17 years at Goldman Sachs Group Inc., becoming partner and head of the mortgage department before working with Hank Paulson in 1999 as the firm's Chief Information Officer.
Mnuchin cashed in on the Goldman Sachs' IPO in 2002 and left to join Lampert's ESL Investments hedge fund as vice chairman he knew Eddie Lampert (whose equity firm owns Sears Holdings and K-Mart) from his time at Yale, where they were roommates.
In 2003 he started new fund with $1 billion from George Soros. In 2004, with two other ex-Goldman Sachs partners, he combined his fund holdings to form hedge fund Dune Capital Management, which led a group (which also included George Soros) that in 2009 raised $1.55 billion to acquire what was left of collapsed mortgage loan bank IndyMac from the FDIC receivership it was renamed OneWest Bank, and in 2015 acquired by CIT Group for $3.4B.
Partly due to this purchase he was subjected to 2011 protests on the lawn of his Bel Air mansion by foreclosed homeowners.
Mnuchin's father, Robert Mnuchin, was a partner at Goldman Sachs in the 1960s.
Steven Mnuchin and Donald Trump have known each other for 15 years and have done business together, though Mnuchin declined to comment on the nature of their dealings and also declined to comment on Trump's attacks on the "carried-interest" that many hedge fund and private equity managers use to lower their taxes.
Mnuchin is a registered Republican but like Trump, he has given money to Democrats, including about $7,000 to Hillary Clinton for her U.S. Senate bids and 2008 presidential campaign. He has also given a substantial amount to Republicans, including $20,000 to the RNC. But the Democratic donations exceeded the ones to Republicans, according to the opensecrets.org - Steven Mnuchin, NY.