The Back-and-Red capitalized on back-line miscues from the hosts to remain first in the East.
All of the game's goals were scored in the second half. Nick DeLeon put the visitors ahead in the 54th minute. Coming in from the right, DeLeon danced around the TFC defense and put a left-footed shot inside the near post.
Luke Moore brought it to 1-1 on the hour mark. Jermain Defoe made it to the endline and cut back a cross to Jackson, whose 18-yard shot was blocked and spilled by Hamid, and Moore was first to the loose ball to level the score.
United's winner came from Perry Kitchen in the 69th minute, with the midfielder eluding his markers and heading in Lewis Neal's corner kick.
Toronto's problems worsened when Nick Hagglund was ejected in the 88th minute. The rookie was initially shown a yellow card when he hauled down Eddie Johnson on a breakaway, but after a conference with his assistant, referee David Gantar upgraded the offense to a red.
D.C. remains first in the East with an 8-5-4 record, although Sporting KC could overtake the capital club on goal difference with a win over Chicago on Sunday. Toronto is in fourth place with at 6-5-3.