Mikael Renberg and Gary Roberts scored for the Maple Leafs, who are still first in the Eastern Conference with 61 points despite winning just two of their last six games.
The Philadelphia Flyers, who lost Tuesday, are second with 59.
Belfour, rarely tested in the first two periods, recorded his 70th career shutout, three fewer that New Jersey's Martin Brodeur on the active list.
The Maple Leafs lost yet another player to injury when Darcy Tucker didn't return after New York's Jason Blake high-sticked him in the face near the end of the second period. Tucker went after Blake after initially dropping to the ice.
The Maple Leafs are already missing forwards Owen Nolan, Alexander Mogilny, Robert Reichel and Wade Belak as well as defenseman Ken Klee.
Nolan missed his eighth straight game after being high-sticked in the left eye on Jan. 3. He expects to return next Tuesday.
Blake didn't receive a penalty for high-sticking Tucker, yet six of the nine penalties called in the game were for high-sticking.
Toronto managed just four shots in the first period, but took a 1-0 lead on Renberg's one-timer from the faceoff circle at 9:02.
New York's Mattias Weinhandl hit the post with a shot midway through the second.
The Islanders managed just one shot in the first 10 minutes of the third, but 11 in the final 10 minutes.
Belfour stacked the pads and robbed Blake of a goal with six minutes left. Fans chanted 'Eddie!, Eddie!' after the save.
Toronto's Mats Sundin made a pass from behind the net to Roberts, who gave Toronto 2-0 with 2:39 left. Renberg also assisted.
The Islanders emptied their net with just over a minute left.
New York ended Toronto's 16-game point streak on Dec. 27 at Nassau Coliseum. ... Toronto defenseman Tomas Kaberle returned after missing five games with a shoulder injury. ... New York's Alexei Yashin and Mark Parrish are sidelined with injuries. ... Toronto lost consecutive games to Philadelphia this past weekend.