New York was forced to start Mikko Koskinen in his NHL debut after second-string goalie Kevin Poulin appeared to injure his left leg during pregame warmups, minutes before the opening faceoff.
Colby Armstrong, who had two assists in the third, and Jay Rosehill scored to give Toronto a 2-0 lead on the Maple Leafs' first two shots at the 22-year-old Koskinen.
Nikolai Kulemin stretched it to 4-2 with 11:11 left and Fredrik Sjostrom made it a three-goal lead with 5:07 remaining. James Reimer made 31 saves for Toronto, which won on the road for the first time since Jan. 11.
The Maple Leafs are 4-1 in an eight-day stretch following the All-Star break. Toronto had plenty of jump in its legs one night after beating Atlanta 5-4. Kulemin also scored in the third period on Monday.
Matt Moulson and Michael Grabner had goals to get the Islanders even, and Rob Schremp added one with 1:52 left, but it wasn't enough. Koskinen and Poulin are up from the AHL only because No. 1 goalie Rick DiPietro is injured and Dwayne Roloson was traded to Tampa Bay earlier this season.
Koskinen made 21 saves.
Poulin fell awkwardly in the crease after he seemed to catch his left skate in a rut. He was helped to the dressing room. New York recalled Joel Martin from Bridgeport of the AHL, and the 28-year-old goalie was dressed and on the bench in the third period.
Koskinen and the Islanders seemed to be store for a very long night early as the Maple Leafs scored on the first two shots the 6-foot-5 Koskinen faced in the NHL.
Armstrong beat the hulking Finn just 1:29 in with a shot from the middle of right circle, and Rosehill made it 2-0 only 5:02 later. Koskinen could hardly be faulted for that one as Rosehill deftly got his stick on the puck in the slot to deflect in defenseman Luke Schenn's drive from the right point for his second career NHL goal.
Koskinen then appeared to settle down and kept the Islanders in the game long enough to get even.
Moulson started the comeback with his team-leading 19th goal, that was set up by a brilliant pass from John Tavares. The young center had his back to the middle of the ice as he dug the puck out of the deep left corner, and sent a blind backhanded pass in front to Moulson, who quickly fired a shot past Reimer with 9:12 left in the first.
Koskinen got a chance to show off his skills a bit in the second period, first denying Clarke MacArthur's quick shot in-close during a 2-on-1 break - a save that left him with splattered snow on his left shoulder - and then doing the splits to make a lunging glove grab of Armstrong's drive.
Grabner, who was stopped on a short-handed breakaway in the second, tied it 2-2 with 4:58 left in the period.
Notes: Rosehill scored his other NHL goal on Oct. 10, 2009, against Pittsburgh. For him, that was a span of only 22 games. ... Armstrong has 13 career assists against the Islanders, his highest total for an opponent. ... Islanders LW Trent Hunter, who had already missed 30 games because of a left knee injury, was ruled out for the season. New York came in with 332 man games lost to injury this season. ... The Islanders had won three straight and nine of 12 against Toronto. ... After earning his first NHL shutout on Thursday, Reimer was pulled on Saturday after allowing five goals on 30 shots in two periods of a 6-2 loss at Buffalo.