Club America, after a 1-1 draw in the first leg last week, won the two-game aggregate goals final 5-3.
It was America's first victory in the Champions League era (since 2009), but the Mexico City side had won the previous version of the competition five times. They will take part in the Club World Cup in Japan in December.
Andres Romero and substitute Jack McInerney scored for Montreal, which was the first Canadian team to reach the final after dramatic wins in the quarter-finals over Mexico's Pachuca and Costa Rica's Alajuelense.
A capacity crowd of 61,004 had Olympic Stadium vibrating even before the kickoff as the Impact fans' "Allez Montreal" song drowned a sizable contingent of America supporters.
Mexico's biggest club were perhaps dazzled by the atmosphere early on, but dominated possession and carved Montreal's defence for four goals in the second half.
Montreal went into the match without suspended goalkeeper Evan Bush. German Kristian Nicht, acquired two days earlier from Indy Eleven of the NASL, could not bail out his overwhelmed squad.
The Impact got the early goal they wanted when Ignacio Piatti dribbled through two defenders on the right side and fed fellow Argentine Romero, who took a few steps to the left and then put a low shot across his body that beat goalkeeper Moises Munoz in the eighth minute.
America missed a golden chance to tie it in the 12th when Darwin Quintero's free kick was headed from the left side to a wide open Benedetto, only to see his close range shot hit the woodwork.
Piatti was stopped from close range when he went in alone in the 24th.
America stormed out after the intermission and equalized in the 50th off a turnover. Osvaldo Martinez chipped the ball over a stagnant Montreal defence and Benedetto scored on an impressive volley that Nicht got his hands on but couldn't hold on to.
Quintero headed a ball that Peralta nodded home from close range in the 64th. Only two minutes later, Bendetto charged up the middle and volleyed home a Miguel Samudio feed.
Then Benedetto scored on an open shot from near the penalty spot in the 81st when the Montreal defence overran the ball.
Piatti slipped a pass to the onrushing McInerney for a goal that brought the crowd back to life in the 89th.
Notes _ With both Victor Cabrera and Hassoun Camara injured, Nigel Reo-Coker was moved to right back and Marco Donadel went in at holding midfielder. . . The record for soccer at the Big O is 71,617 for East Germany's 3-1 win over Poland at the 1976 Olympics.