The source told CNN that Mnuchin could be in "serious danger" with Trump, who regularly raids Cabinet members whom he mistreated before cooling down.
Nevertheless, Trump publicly vouched for Mnuchin, shifting the debt instead to market volatility against the Federal Reserve.
But the source drew another picture of Mnuchins behind the scenes.
"Mnuchin is under the gun," the source said.
The US Treasury Secretary left Washington for Christmas in Cabo San Lucas, Mexico, when the federal government closed on the weekend, while Trump canceled his own planned trip to his Mar-a-Lago resort in Florida and Washington state The White House held on to the holiday and absorb a flood of negative news about the markets.
Mnuchin's aides searched for economic data to reassure their boss to reassure Trump, but Trump was unhappy with what Mnuchin told him. this source said.
One administration source dismissed the last round of rumors that the secretary's term of office continued.
"That's nonsense," they said.
Over the weekend, the secretary telephoned the president several times, even after a brutal trading day on Christmas Eve, leading to a 653-point decline in the Dow Jones Industrial Average, sources in the administration said. The two men spoke on a variety of topics, including the closure of the government and an update on the state of trade talks.
Mnuchin, a former investment banker and film financier, has long been one of Trump's closest advisers, dating back to him working with Trump on the 201
For once, Wall Street was volatile throughout December, and investors were already jumping over signs of a global slowdown that was repeatedly shaken by Trump's own comments. The chute began in earnest with the President's own tweets after announcing a trade deal with Beijing in which he proposed to impose radically higher tariffs if he was not satisfied with negotiations.
But his efforts to calm the volatile markets for weeks seemed to be failed on Monday.
The Dow fell 2.9% and the S & P 500 2.7%. This was the biggest decline of the two indices on Christmas Eve. On the same day, Trump fired a series of tweets, including an attack on the Federal Reserve. [Christmas Day] Trump said he remained confident with Mnuchin but still was not satisfied with the Fed.
"They raise interest rates too fast," said Trump of the Central Bank.
Kevin Liptak of CNN contributed to this report.