Substitute Andres Romero scored the deciding goal as the Impact advanced on away-goal aggregate despite falling 4-2 to Costa Rica's Alajuelense on Tuesday in the second leg of their two-match semifinal series.
Romero evaded a couple of Alajuelense defenders before burying a shot from 12 yards out in the 72nd minute.
Montreal came into the match on a 2-0 aggregate lead thanks to a victory in the opening leg March 18 at Olympic Stadium on goals from Ignacio Piatti and Victor Cabrera. The final aggregate score was 4-4, with more weight given to goals scored on the road.
"The score line isn't important for me," said Impact defender Bakary Soumare. "It's the result that counts. We were here to get to the next round and we did what we needed to do that. ... We are very happy with what we've been able to accomplish and now our goal is to win the cup."
Jack McInerney also scored for the Impact, taking a perfect feed from Dominic Oduro and firing a shot to the left of a sprawling Dexter Lewis Bonilla in the Alajuelense goal in the 42nd minute.
Pablo Gabas scored twice for Alajuelense, who made things interesting with four second-half goals, substitute Allen Guevara Zuniga found the back of the net in the 79th minute and Jonathan McDonald added a stoppage-time goal.
"We knew we were coming to a difficult place to play with a tough team," said Impact head coach Frank Klopas. "They have a tremendous push with the quality of players they have and their fans. We scored an away goal, which was very important. The quick goal in the second half brought them back into the game. It was a great semifinal and a great moment for our club, the city of Montreal and Major League Soccer."
The Impact will play either Costa Rica's Herediano or Club America of Mexico in the final. Herediano beat America 3-0 in their first leg, with the second leg scheduled for Wednesday in Coyoacan, Mexico.
Montreal is only the second MLS team, after Salt Lake in 2011, to reach the CONCACAF final since the Champions League started in 2008-09.
The final will take place Apr. 22 and 29.
Out with long-term injuries are midfielder Justin Mapp (fractured elbow) and forward Cameron Porter (torn ACL).