MacArthur tallied 1:43 into the contest, and Nazem Kadri and Carl Gunnarsson then scored to give Toronto a 3-0 cushion.
Kadri scored for the second time in as many nights after being called up from the minors. The goal for Gunnarsson, a defenseman, was his first of the season and just his eighth in 145 NHL games.
The Islanders twice battled back to within a goal, but Joffrey Lupul clinched the victory by scoring into an empty net with 9 seconds remaining in the third period.
James Reimer got the win with 30 saves.
Evgeni Nabokov continued to struggle, giving up four goals on 26 shots. He has given up at least three goals in five of his last six starts and his record fell to 2-7.