McInerney opened the scoring in the 7th minute off a through-ball from Conor Casey and found the net again in the 26th, handing Philadelphia a 3-1 advantage.
Casey also tallied in the first half for the Union (3-2-2), who triumphed for the first time at RFK Stadium while snapping a three-game winless streak.
Perry Kitchen and Lionard Pajoy each scored for D.C. United (1-5-1), which fell for a fourth straight time.
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