CF Montreal (19-9-5, 62 points) improved to 10-1-3 over their past 14 matches and moved within two points of the Eastern Conference-leading Philadelphia Union with one regular season match left for both clubs.
The D.C. United own goal was credited to Donovan Pines, who later got a red card to leave his team playing a man short for the final 11 minutes of regulation and four stoppage-time minutes.
D.C. United (7-20-6, 27 points) dropped to 0-3-2 over their past five matches. D.C. United has the fewest points in the 28-team league.
D.C. United's Ted Ku-Dipietro found the back of the net in the 87th minute but the goal was waved off due to Steven Birnbaum being offside.
James Pantemis made two saves while recording his fourth shutout of the season for Montreal.
The own goal came after Montreal's Zachary Brault-Guillard delivered a cross toward the net in the 41st minute. D.C. United goalkeeper David Ochoa was sliding over to collect the ball, but Pines tried to clear it and instead he knocked the ball past the goalie and into the net.
Montreal had a chance to add to the lead in the 80th minute when Joaquin Torres lined up for a penalty kick. But Ochoa was ready and dove to his left to make the save on Torres' left-footed kick.
But D.C. United played a man short the rest of the contest because the penalty kick was awarded after Pines was given a red card in the 79th minute for pushing down Montreal's Ismael Kone in the box three minutes earlier.
Ochoa had three saves for D.C. United.
D.C. United played without leading scorer Taxi Fountas (12 goals).
The Greek hasn't returned to the club after playing internationally for his country earlier this week in the wake of an allegation that he hurled a racial slur at Inter Miami CF's Damion Lowe, who is Black, in a match on Sept. 18. Fountas has strongly denied the accusation.