When Craig S. Phillips, a veteran Wall Street mortgage trader, agreed to join the Treasury Department as a top aide to Secretary Steven Mnuchin in 2017, his mandate was to overhaul the way that American home loans are guaranteed and then traded by the federal government.

More than two years later, with that ambitious agenda far from fruition, Mr. Phillips is returning to the private sector.

Mr. Phillips told Treasury Department employees on Thursday that he planned to leave the government next month, according to a person familiar with the conversation who was not authorized to speak publicly. He informed colleagues that he expected to return to Wall Street but had not lined up a new job.

During his two and a half years at Treasury, Mr. Phillips helped create a blueprint for relaxing financial regulations imposed after the 2008 financial crisis.