"The US feels very strongly that any tax that is designed specifically on digital companies is a discriminatory tax and is not appropriate", Mnuchin said.
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.
Mnuchin rushed to respond. "If you DO have one, as I do, they'll claim it's illegitimate".
"Just think for a moment, what good is all the extra wealth in the world gained from business as usual if you can do nothing with it except watch it burn in catastrophic conditions".
He claimed that countries other than the United States were the worst polluters and "Greta ought to focus on those places".
The summit broke up with most delegates agreeing there's a big problem but little consensus on what should happen next.
In a speech on Tuesday, US President Donald Trump had castigated the "prophets of doom" and those that predicted a climate "apocalypse", in comments widely seen as an attack on Thunberg who sat in the audience. At Friday's news conference, Thunberg shared the spotlight with Vanessa Nakate of Uganda, Loukina Tille of Switzerland, Luisa Neubauer of Germany, and Isabelle Axelsson of Sweden.
"Of course those demands have been completely ignored, but we expected nothing less", she said at a press conference with four other youth activists.
He called for 2020 to be "the year that we put ourselves on the right track" and for the private sector to "lead the world out of the approaching catastrophe into which we have engineered ourselves".
'And this is going back to the president during the campaign, he said, post-Brexit, they'll be at the top of the list'.
He said: "We believe that DST is a discriminatory tax and discussions are ongoing on that". "Does that pose a threat to U.S. economic growth?"
Thunberg repeated her climate warnings, hammering home in her speech that it was time to "panic" because "the house is burning", as she did the previous year in Davos.
Mr Mnuchin, who met Mr Javid for breakfast on Saturday morning and posted an image of them on Instagram, said the U.S. was "prepared to dedicate a lot of resources" to securing a trade deal with the United Kingdom this year. "Waiting for the others is a possibly very risky strategy".