The Black-and-Red did all the damage in the first half as Luis Silva grabbed three goals and Fabian Espindola picked up three assists.
A Luis Silva hat trick led D.C. to a 4-2 lead by the end of the first half. He opened the scoring with a sixth-minute finish, with Fabian Espidola picking out his diagonal run to cap off a good D.C. move.
Jack McInerney pulled the Impact even in the 12th minute, sending Hassoun Camara's through ball just past Andrew Dykstra's fingers into the low far corner. Montreal then went 2-1 up in the 21st minute. Andres Romero weaved his way across the top of the box and in to drive home his shot, leaving a number of defenders in his wake.
The lead was short-lived. Nick DeLeon turned in a brilliantly played low Espindola cross from wide to level it up. Another Silva goal, a perfectly placed shot from distance when the Montreal defense had given him too much room, returned control to the visitors.
Silva's hat trick came in first-half stoppage time. Camara was whistled for a foul in the box, and the midfielder converted.
Montreal had a second-half penalty in the 75th minute, but Patrice Bernier's effort hit the post.
Montreal falls to 2-7-4 heading into the World Cup break, and the Impact have the worst record in MLS. United leads to the top of the Eastern Conference with its 7-4-4 record.