The game's only goal gave Eastern Conference-leading United (9-4-3) its fourth straight victory.
D.C. had to weather a strong assault by the Union on the first half, when they put nine shots on goal, forcing three saves by goaltender Bill Hamid.
Rookie Antoine Hoppenot took a through ball from Freddy Adu in the 62nd minute but banged it off the post for Philadelphia (2-8-2), which lost in its first game under new Coach John Hackworth.
The Union remained winless in six games.
Susan Sarandon 'very excited' about daughter's pregnancy
Scarlett Johansson steps out with fiance after pregnancy reveal