He was sworn in as the Treasury Secretary by Vice President Mike Pence soon after he was confirmed by the US Senate by a vote of 53 to 48 yesterday.
Mnuchin, 54, is the third former Goldman Sachs executive to become Treasury Secretary. The other two being Robert Rubin and Henry Paulson.
In this position, Mnuchin is expected to play a key role in the economic reforms that Trump plans to undertake to remove bureaucratic bottlenecks, simplify tax codes and make America an attractive investment destination.
"With decades of private-sector experience, Secretary Mnuchin understands how today's broken tax code and wasteful spending directly discourage economic growth and make it harder for Americans to get ahead," he said.
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However, a number of Democratic lawmakers opposed the nomination of Mnuchin as the Treasury Secretary.
"The American people don't suffer from the same collective amnesia that Wall Street and its allies in Congress have about how devastating the crisis was to our country," Senator Sherrod Brown said on Senate floor.
"As the people we represent know, and Mnuchin's bank proves, when we turn the reins to Wall Street, it's working families who pay the price," he said.
Mnuchin will be the principal economic advisor to Trump on domestic and international financial, economic, and tax issues.
"Regrettably, while Mr Mnchuin may have a knowledge of the inner-workings of Wall Street, he seems to know shockingly little of the hardships faced on Main Street. One need look no further than his role during the height of the housing crisis in foreclosing on tens of thousands of American families," Senator Patrick Leahy said.
"Reducing these actions to mere administrative matters belies the true struggles of those who don't boast the personal coffers Mr Mnuchin enjoys. I simply cannot accept his explanation of his role in these actions," he said.
Mnuchin is responsible for the executive branch agency whose mission includes maintaining a strong economy, promoting conditions that enable job growth and stability, protecting the integrity of the financial system, strengthening national security, and managing the US government's finances effectively.
He succeeds Jacob J Lew, who served in the position under former US President Barack Obama.
Prior to his confirmation, Mnuchin was Finance Chairman for Donald Trump for President. In this role he spent the last year travelling with Trump, met with hundreds of business leaders. He also served as a Senior Economic Advisor to the President in crafting his economic positions and economic speeches.
Earlier in his career, Mnuchin worked at The Goldman Sachs Group, where he was a Partner and served as Chief Information Officer. He has extensive experience in global financial markets and oversaw trading in US government securities, mortgages, money markets, and municipal bonds.
Senator Tim Kaine said he was not confident Mnuchin will be an advocate for working families and their financial security.
Trump said Mnuchin would be a champion of tax payers, would fight for the middle class and ensure that the Wall Street plays by the rules.
"To all citizens I say, Steven will be your champion, and a great champion. He will fight for middle class tax reductions, financial reforms, and open up lending and create millions of new jobs, and fiercely defend the American tax dollar and our financial security," Trump said during the swearing in ceremony of Mnuchin in the Oval Office of the White House.
"And he will also defend our manufacturing jobs from those who cheat and steal and rob us blind. It won't be that anymore, we won't have that anymore. Countries and others won't be able to take advantage of us, it's a whole new era," said the US President.
The new Treasury Secretary he said will put his skills to work for America's working families and he will help make America the greatest jobs magnet on the face of the earth.
"It will truly be for our country, for the first time in a long time, a rising tide that lifts all boats and that lifts all ships," Trump said.
In his brief remarks, Mnuchin said he would carry Trump's economic vision and goals.
"I am committed to working with you to enact policies to grow the economy and make better economic opportunities for all Americans," he said.