Toronto (19-10-1) is 11-3-1 in its past 15 games, including 5-2-0 in its past seven.
Pittsburgh (16-12-3) has lost two of its past three games (1-2-0) after winning its previous four.
"We just weren't good enough and it was the difference in the outcome," Penguins coach Mike Sullivan said. "I mean, we can't keep chasing games the way we're chasing them, and when you play good teams you can't spot them a three-goal lead. When you give up a couple of goals so early in a game, it's a head-scratcher for me.
"I'm not sure I understand why, but that's something that we as a group have to solve if we're going to become the team we want to be."
The Maple Leafs scored three first-period goals, including two in the first 1:42. Bozak had the secondary assist on the second goal and scored the third.
"Real good for [Bozak]," Maple Leafs coach Mike Babcock said. "Things haven't gone as well for him this year and confidence is always hard. … Worked real hard. Competed real hard."
Connor Brown gave the Maple Leafs a 1-0 lead after receiving a pass from Morgan Rielly, which he shot through a screen from Zach Hyman at 1:23. The goal ended his eight-game drought, dating to Nov. 18.
Van Riemsdyk made it 2-0 19 seconds later.
Tristan Jarry stopped a backhand shot from Bozak before Marner retrieved the puck behind the net and attempted a wraparound. Chad Ruhwedel knocked the puck off his stick, but allowed a pass from Marner through his legs to van Riemdyk, who sent a wrist shot past Jarry at 1:42.
Bozak extended the lead to 3-0 after a shot from Marner popped out of Jarry's glove, allowing Bozak to whack at the rebound twice before scoring on a third attempt at 12:55.
DeSmith replaced Jarry to start the second period. Jarry made 13 saves on 16 shots; DeSmith made eight saves on nine shots.
Frederik Andersen made 33 saves for Toronto.
"We were opportunistic in the first period," Andersen said. "Not as experienced of a goalie [Jarry] down there, so we wanted to make sure he saw a lot of pucks and go to the net."
