The idea of putting the former slave and abolitionist on the bill was proposed during the Obama administration.
Mnuchin told CNBC on Thursday: "Ultimately we will be looking at this issue. It's not something I'm focused on at the moment."
Instead, he said he was concentrating on making sure any tweaks to U.S. currency bills thwart counterfeiters.
"The No. 1 issue why we change the currency is to stop counterfeiting. So the issues of what we change will be primarily related to what we need to do for security purposes. I've received classified briefings on that. And that's what I'm focused on for the most part," Mnuchin said.
"People have been on the bills for a long period of time. And this is something we will consider. Right now, we've got a lot more important issues to focus on," he added.
As part of the April 2016 announcement, Lew said Alexander Hamilton would stay on the $10 bill, which had been a possible home for Tubman's image. (That decision was well-received by fans of Lin-Manuel Miranda's Broadway smash-hit musical "Hamilton" who wanted the nation's first Treasury secretary to stay put.)
In 2016, during the presidential campaign, candidate Donald Trump told NBC's "TODAY" show, "I think Harriet Tubman is fantastic." He added: "I would love to leave Andrew Jackson and see if we can maybe come up with another denomination. Maybe we do the $2 bill or we do another bill."
While Trump complimented Tubman, he said at the time that he didn't agree with replacing Jackson on the denomination. "I don't like seeing it. Yes, I think it's pure political correctness. Been on the bill [Jackson] for many, many years. And, you know, really represented somebody that really was very important to this country.
President Trump has made no secret of his admiration for Jackson. He even put a portrait of Jackson in the Oval Office. In past tweets, Trump has given a shoutout to Jackson.
Mnuchin's remarks on Thursday sparked discussion on Twitter, including a tweet from Chelsea Clinton, daughter of former President Bill Clinton and Hillary Clinton, the 2016 Democratic presidential nominee.
Meanwhile, Symone Sanders, former national press secretary for the campaign by Vermont Sen. Bernie Sanders for the 2016 Democratic presidential nomination, was outraged.
As mentioned, the Democratic nomination went to former Secretary of State Clinton.
In Thursday's CNBC interview, Mnuchin also said the administration has a "very detailed" tax plan ready and "couldn't be more excited" about its prospects.
Mnuchin weighed in on the dollar as well, saying a weaker dollar puts the United States in a better position on trade.