Mnuchin was an executive producer on the movie, which is a spinoff of 2014's The Lego Movie.
Be Civil - It's OK to have a difference in opinion but there's no need to be a jerk. "So I just want to have the legal disclosure, you've asked me the question, and I am not promoting any product", he said.
In a letter to the Office of Government Ethics (OGE), Mnuchin explained that when he promoted the movie during an interview with Axios, it was not his "intention to promote" the movie. "I should not have made that statement".
Lego Batman is so far the third-highest grossing film of 2017, having taken in US$171.7 million (S$24.7 million) since opening last month, according to Box Office Mojo.
Asked then about his Hollywood output, Mnuchin said, "Well, I'm not allowed to promote anything that I'm involved in".
Federal ethics rules prohibit executive branch employees from using their position for personal gain or "for the endorsement of any product, service or enterprise, or for the private gain of friends, relatives or persons with whom the employee is affiliated in a nongovernmental capacity".
He added, "I want to reassure you that I will exercise greater caution to avoid any suggestion that I do not take these important rules seriously".
The office's director warned there was "strong reason" to believe Conway violated ethics rules and said "disciplinary action was warranted".
Sean Spicer, the White House press secretary, said in February that Conway had been "counseled" about her comments.