Roared on by a fervent home crowd at BMO Field, Toronto made sure of a first ever MLS Cup appearance against Seattle Sounders thanks to extra-time goals from substitutes Benoit Cheyrou and Tosaint Ricketts.
Defeat will sting for Montreal and veteran striker Didier Drogba, who brought the curtain down on his time with the club as a second-half substitute, given it had cruised into a 3-0 lead in last week's first leg before being pegged back to 3-2.
Impact struck first on the night once more, with ex-Toronto striker Dominic Oduro on target against the run of play after Michael Bradley dallied in possession.
But poor set-piece defending towards the end of the first half would herald the beginning of Montreal's downfall, as Armando Cooper prodded home before the excellent Jozy Altidore powered to the near post to nod in from Sebastian Giovinco's corner.
An onslaught appeared to be on the cards but Impact restored its aggregate advantage eight minutes after the break when Ignacio Piatti scrambled in despite Oduro appearing to be in an offside position.
Nick Hagglund headed in superbly from Justin Morrow's cross in the 68th minute and Drogba's introduction could not avert extra-time and its associated heartbreak for Montreal.
Cheyrou came on for a hobbling Giovinco and made an instant impression, scoring with his first touch in the 98th minute.
With Montreal still reeling, Altidore tormented it once more, tearing down the right flank and settling up Ricketts to get the party started.