Kamara converted a penalty kick in the 30th minute and added his team-leading 19th goal six minutes later for D.C. United (14-15-5, 47 points), who were denied entry into the playoffs despite the victory.
In addition to securing a win, D.C. United needed Nashville SC to defeat the New York Red Bulls on Sunday and have CF Montreal either lose or tie versus Orlando City SC in order to make the playoffs. Montreal dropped a 2-0 decision, however Nashville SC's 1-1 draw eliminated D.C. United.
Richie Laryea scored in the 18th minute and Quentin Westberg finished with six saves for Toronto FC (6-18-10, 28 points), who already had been eliminated from the playoffs and fell to 0-3-3 in their last six matches.
Toronto FC also dropped a 7-1 setback to D.C. United on July 3, a result that led to then-coach Chris Armas being fired the following day.
Kamara drew a foul on Westberg in the penalty area in the 29th minute and converted the subsequent penalty kick one minute later to give D.C. United a 2-1 lead.
D.C. United opened the scoring in the 13th minute after Birnbaum's headed home a corner kick from Gressel. The goal was Birnbaum's second of the season and first since Sept. 25.
Laryea forged a tie five minutes later with a shot from outside of the box that sailed just inside the left post. Laryea's goal was his third and first since Aug. 7.