The Islanders had a pair of leads in Sunday's game, but saw 1-0 and 2-1 advantages get wiped out by quick comebacks from the Maple Leafs.
Anthony Beauvillier opened the scoring with a power-play goal at 14:17 of the first period. Beauvillier, who snapped a nine-game point drought, had plenty of time and space at the right dot, beating a screened Jack Campbell far side.
The 1-0 lead didn't last long, as Mitch Marner evened the score 3:14 later, knocking in a Mark Giordano rebound after Ilya Sorokin made a quality first save on the Leafs defenseman. Marner's goal was his fourth in three games against the Islanders this season.
The Islanders took a lead early in the second period, as Alex Kerfoot swatted at a puck in front of the net that turned into an own goal 31 seconds into the frame. Bailey was given credit for the unassisted goal.
The Leafs, who came back from 2-0, 3-1 and 4-3 vs Ottawa on Saturday night, took the lead before the end of the period. Pierre Engvall finished a two-on-one rush with Kerfoot to tie the score at 10:49, while William Nylander put Toronto ahead with a power-play goal at 13:20. Nylander had all sorts of time and space in the Islanders zone after a botched clearing attempt at the blueline took the Islanders out of their structure against the league's top power play.
The Isles penalty kill did come up with a big 1:21 kill during a Leafs' two-man advantage to give New York a chance. That allowed the Islanders to stay in the game, but they could not convert. Kyle Palmieri was robbed by a sprawling Campbell and Barzal, who saw an extended shift with Anders Lee and Brock Nelson, also had a quality look.
Toronto outshot the Islanders 16-9 in the final frame with David Kampf eventually icing it with an empty-netter.
With the loss, the Islanders were swept in the season series vs Toronto.