WASHINGTON — Treasury Secretary Steven Mnuchin flew from Washington to Los Angeles this month on the private aircraft of Michael R. Milken, the billionaire “junk bond” king who pleaded guilty to securities fraud in 1990 and served two years in prison.

The flight, which was confirmed by the Treasury Department on Friday, was the latest example of Trump administration officials using luxury or government aircraft for personal reasons. Mr. Mnuchin, who was accompanied by Secret Service agents on Mr. Milken’s jet, travels frequently to California to visit his children who live there.

A Treasury Department spokesman said that Mr. Mnuchin and Mr. Milken have known each other for years and that after reviewing internally the secretary’s plans to take the flight, it was decided that he did not need an ethics waiver. Mr. Mnuchin has reimbursed Mr. Milken for the cost of the flight, the spokesman said, but did not disclose the amount.

Trump administration officials, including Mr. Mnuchin, had been encouraging President Trump last year to pardon Mr. Milken, who pleaded guilty to six criminal charges related to securities transactions undertaken in the 1980s. Mr. Milken, who had to pay $600 million in fines, was sentenced to 10 years in prison and released after two years.