In a wild playoff game postponed for 24 hours by snow, D.C. United survived a missed penalty kick and both teams had a player sent off in the second half.
D.C. United won the two-match, aggregate score conference semifinal series 2-1 and will now meet Houston for the conference title. Houston will host the first game of the two-leg series on Sunday.
Thursday's game was originally set for Wednesday on New York's home field, but heavy snow eventually forced a one-day delay.
Once the game finally got underway it appeared destined for overtime, but a New York defensive miscue led to the lone goal and a D.C. United win.
A high bouncing ball eluded three New York players in the defensive end, allowing D.C. United to set up its offense.
Robbie Russell eventually wound up with the ball just outside the penalty area and his through ball past three New York defenders wound up on the foot of DeLeon. He quickly fired it past New York goalkeeper Luis Robles.
New York missed a tremendous chance to go in front in the 69th minute when Houston goalie Bill Hamid took down Kenny Cooper in the penalty area with a hard foul.
Hamid drew a red card, leaving D.C. United a man short, and Joe Willis came in to replace Hamid in goal.
Cooper made his penalty kick, but it was waived off by referee Mark Geiger because one of the New York players encroached too close to the goal. Cooper's second attempt, driven to the bottom right corner of the net, was stopped by Willis.
New York also lost a man in the 75th minute when Rafael Marquez picked up his second yellow card of the game.