The total of the underpayments were more than $1.4 million.
The DOJ said he did it in exchange for cash payments of more than $380,000 and sexual favors, according to WJLA.
McMillan faces up to 15 years behind bars and a fine of up to $300,000 when he is sentenced in June, WRC reported. He also has to repay $1,456,985 and forfeit $74,811 in cash that was confiscated from his home, WJLA reported.
The director of the Department of Human Services said she was "absolutely sickened" by the case, but doesn't believe such behavior was running rampant in the department, WRC reported.