Toronto got on the board first at the 10:27 mark of the opening frame.
Defenseman Jake Muzzin accepted winger William Nylander's cross-ice feed before beating goaltender Cayden Primeau up high to make it 1-0.
That would stand as the only tally of the first, with Toronto also holding a 9-5 lead in shots after 20 minutes.
Sheldon Keefe's squad upped their lead just 1:55 into the second period on the power play.
Captain John Tavares deflected Nylander's shot from the faceoff circle to give Toronto a 2-0 lead.
The Leafs padded their lead less than five minutes later thanks to Michael Bunting.
The 26-year-old winger redirected defenseman TJ Brodie's pass to beat Primeau and make it 3-0.
Primeau vacated the net in favor of Michael McNiven with 6:22 left in the second. Primeau turned aside 18 of 21 shots in his first outing of the preseason.
Montreal was trailing 3-0 on the scoreboard and 24-7 in shots after 40 minutes.
Forward Kurtis Gabriel scored the Leafs' fourth goal of the contest with 4:48 to play in regulation after McNiven's botched attempt to chase the puck at the circle.
Tyler Toffoli broke the Leafs' shutout attempt on a breakaway with less than two minutes to play.