Mnuchin was dismissive of the 17-year-old Swedish climate activist at the World Economic Forum in Davos, Switzerland, and said Thunberg should go study economics in college before weighing in on U.S. policies and how they relate to the climate crisis.
According to New York Times reporter Alan Rappeport, Mnuchin also added that he was not an "expert" on environmentalism.
A day earlier, Mnuchin dismissed Thunberg's suggestion that governments and companies need to cut back dramatically on their use of fossil fuels with a condescending barb.
When asked how that would affect the United States economic model, Mnuchin said of Thunberg: 'Is she the chief economist?
Mnuchin's remarks on Saturday follow a week of intense happy talk by both British and USA leaders over a trade deal, which has come to major prominence during the Davos globalist economics summit, amplified by the looming Brexit date for Britain to leave the European Union.
Isabelle Axelsson is one of the youngest delegates at Davos, where the 19-year-old, dressed in Dr. Martens boots and dungarees, and her fellow climate activists are calling on the world leaders to do more to tackle climate change. "The situation is not being treated like the crisis it is".
Thunberg has also been a target of President Donald Trump, who has mocked the teen activist on Twitter.
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Mr Mnuchin added President Donald Trump had previously said the United Kingdom would "be at the top of the list" for a deal.
"People have their eyes on us", Axelsson said.
"We are being criticized like that all the time", Thunberg said at the news conference onstage with four other campaigners.
At the meeting of global leaders in Davos, Mnuchin was asked to comment on the debate over the economics of climate change spurred by the Swedish teenager. "Christine, I think you can have a lot of people look at this and model it", he said.
But either by accident or design, there was no meeting between Trump and Thunberg at the forum.
Asked by a reporter if Britain's plan to implement a digital services tax on USA technology giants such as Facebook and Google could hinder the trade negotiations, Mnuchin said that he discussed the issue on Saturday with Javid.
But Thunberg, who was reportedly in attendance during Trump's speech, maintained Tuesday that she has no interest in political tiffs.