Left winger Dylan Hunter and right wingers Drew Stafford and Anthony Stewart, with an empty-netter, scored for the Americans in front of 3,777 fans at Ricoh Coliseum.
Right winger Brad Leeb had the lone marker for Toronto, who fell to 2-3-1.
Dennis stopped 22 shots in the final frame and was called on throughout the game to bail out his Rochester teammates. At the other end, Toronto's Justin Pogge made 21 saves on 23 shots.
The Americans went 0-for-3 on the power play, while the Marlies were 1-for-7 with the man advantage.
The game was tied at one after the first period and Rochester led 2-1 through 40 minutes.
The Marlies play their third game in three days when the Peoria Rivermen visit the Ricoh Coliseum on Sunday.