They took control in the final 10 minutes of the first half with two goals in the space of three minutes from Marco Di Vaio and Felipe Martins, before second half strikes from Jeb Brovsky, Di Vaio again and substitute Andrea Pisanu added an exclamation mark to the win.
The result puts the Impact two points clear at the top of the Eastern Conference with 41 points from 24 games.
The New York Red Bulls can go top by a point if they beat Chivas USA on Sunday, but the Red Bulls will by then have played two games more than Montreal.
Real Salt Lake maintained their advantage in the Western Conference with a 4-0 victory over the Columbus Crew, but the Los Angeles Galaxy remain on their heels.
Kyle Beckerman broke the deadlock for Salt Lake on the stroke of half-time, and then capped the win a minute from time after second-half goals from Devon Sandoval and Luis Gil.
The Crew's evening deteriorated further with the last-minute dismissal of Federico Higuain for a second bookable offence.
The Galaxy responded with a 1-0 win over the Vancouver Whitecaps, secured through Landon Donovan's goal only three minutes in.
Salt Lake top the Western Conference by five points, but have played two games more than Los Angeles.
FC Dallas came from 2-0 down to grab a share of the spoils in an ill-tempered 2-2 draw with the San Jose Earthquakes that featured three red cards.
The visitors jumped into the lead with an eighth minute strike from Alan Gordon and Steven Beitashour's 16th minute goal.
But Blas Perez got FC Dallas on the board three minutes later, and it was level through Garbini Michel's penalty in the 69th minute after Victor Bernardez fouled Perez.
In the 81st minute San Jose's Justin Morrow was sent off for a foul on Ramon Nunez, and five minutes later Bernardez and Dallas' Kenny Cooper were both dismissed following a fight, but there would be no more goals.
There was less drama but a similar result in the meeting between DC United and Toronto FC, which ended 1-1, both goals scored by player facing their former clubs.
Dwayne De Rosario gave DC United the lead in the 10th minute before Bobby Convey levelled against his old employers on the hour mark.