"We are actively working on Russia sanctions coming out of the classified briefing," Mnuchin told lawmakers on the Senate Finance Committee, where he was testifying on the President's 2019 budget.
The secretary has come under repeated fire by Democrats for the administration's failure to implement an overwhelmingly bipartisan measure that gives Treasury new authorities to impose sanctions on Russia.
"I can assure you those sanctions are coming," Mnuchin said. "We are working on them as we speak."
The Trump administration didn't levy a single sanction on Jan. 29, the first day it could have under the Countering America's Adversaries Through Sanctions Act. That day, anyone doing business with certain Russian intelligence and military entities, including arms manufacturers, faced possible penalties.
The act, drawn up by lawmakers for the express purpose of punishing Russia for its election interference, also required the Treasury Department to report to Congress with a list of powerful and wealthy Russians close to Vladimir Putin, with a view to targeting the Russian president's powerful allies. Treasury was also required to assemble a classified, more detailed version of the oligarchs list.
The State Department has said it considered Jan. 29 a starting point, not a deadline to impose sanctions, and that it would take time to see whether other countries, including close US allies, are indeed weaning themselves off military contracts with the blacklisted Russian suppliers.
Like Mnuchin, State Department officials have offered no timelines.
Democratic senators pressed the treasury secretary on why Trump had yet to publicly call for penalties against Russia following its meddling -- and if Trump had directed Mnuchin to take further action.
"Has the President of the United States asked you to impose sanctions on Russia?" Sen. Claire McCaskill, a Missouri Democrat, asked at the hearing. "I want to know if he asked you. I've never heard the President of the United States say a bad word about Russia. Ever."
Mnuchin said the President never had to directly ask him, because he had pre-emptively briefed Trump on his plans following the completion of Treasury's report to Congress at the end of January.
"He didn't have to ask me, because I've updated the President," said Mnuchin. "I told him we would be doing sanctions against Russia and he was pleased to hear that."
On Tuesday, Director of National Intelligence Dan Coats and the other top national security officials told the Senate Intelligence Committee that they still view Moscow as a threat to the 2018 elections.
"We expect Russia to continue using propaganda, social media, false-flag personas, sympathetic spokesmen and other means to influence, to try to build on its wide range of operations and exacerbate social and political fissures in the United States," Coats said at a hearing on worldwide threats.
"There should be no doubt that Russia perceives its past efforts as successful and views the 2018 US midterm elections as a potential target for Russian influence operations," he said.
Mnuchin didn't offer any specific timing on when Treasury would take action, but he indicated sanctions were coming soon.
The secretary has previously disputed claims that the Trump administration has punted on moving forward with fresh sanctions against Russia, saying it did "not waive or delay."
Treasury met its statutory deadline in providing to Congress a list of every senior member of the political administration at the Kremlin and every Russian oligarch with a net worth of $1 billion or more. But the list came under criticism as it closely mirrored one assembled by Forbes Magazine that cataloged Russia's richest people.
The list included 114 senior political figures with close ties to Putin, including his chief spokesman, Dmitry Peskov, Foreign Minister Sergey Lavrov and Prime Minister Dmitry Medvedev.
An additional 96 oligarchs with net worths of $1 billion or more were also publicly named in the report. They included the aluminum magnate Oleg Deripaska, Chelsea football club owner Roman Abramovich and the media and tech magnate Alisher Usmanov.
CNN's Jeremy Herb contributed to this report