Nick DeLeon netted an 88th minute winner following the 1-1 draw in the first leg on the weekend in a game which saw both sides finish with 10 men among a plethora of chances, particularly for the home side.
DC goalkeeper Bill Hamid was given his marching orders with 20 minutes remaining for denying Kenny Cooper a clear goalscoring opportunity before Rafael Marquez received a second yellow card in the 75th minute in what was a wild finish.
Hamid's foul resulted in a penalty which Cooper tucked past replacement gloveman Joe Willis but referee Mark Geiger ordered a re-take for encroachment of the area with Cooper then having the subsequent spot-kick saved.
Nevertheless, DC moved through to the playoff final and match-winner DeLeon was over the moon during the post-match.
"It's unbelievable. I'm speechless," DeLeon said. "Just being a part of this group, it's been a great journey and it's not over yet.
"The fans, man. I can't thank them enough for coming out. This is for them, this is for our franchise, this if for everybody.
"We came out today to try to prove something about the credit we deserve. We're going to keep proving people wrong. That's what we're here to do."