Kitchen broke the deadlock in the 70th minute, scoring on a header off a corner kick.
Earlier, DeLeon opened the scoring in the 54th minute, before Luke Moore drew Toronto level on the hour.
Bradley had a good scoring chance in the 34th minute, but the U.S. midfielder's shot went wide.
"I felt good, excited to be back," Bradley said. "I'm disappointed that the game went the way it did, because this was a big game against a team close to us in the table and it would have been nice to capitalize."
United improved to 27 points atop of the Eastern Conference, while Toronto has 23 points.
Sebastian Fernandez's long range strike in the 12th minute lifted the Vancouver Whitecaps to a 1-0 victory over the Seattle Sounders.
Fernandez took a pass from Pedro Morales and launched a shot about 30 yards out that sailed past goalkeeper Stefan Frei. Major League Soccer-leading Seattle remained on 35 points, while Vancouver improved to 25 points in the Western Conference.
In Los Angeles, Erik Torres scored in second half injury time to snatch a 1-0 victory for Chivas USA over Montreal.