John Tavares scored, and Jack Campbell made 24 saves for the Maple Leafs (7-5-1), who had their winning streak end at five games.
"We just weren't able to sustain our game," Tavares said. "They do a really good job in the neutral zone and don't give us a whole lot, even just to get good efficient dumps where we can recover pucks. Giving up the lead and having to play from behind, we couldn't seem to make enough plays and be consistent enough putting pressure on them."
Trevor Moore scored his first of the season to put Los Angeles ahead 1-0 at 10:52 of the first period. He skated past Timothy Liljegren down the right wall, cut inside on Rasmus Sandin and chipped a shot over Campbell's glove from the top of the crease.
Athanasiou made it 2-0 at 14:21 when he got around Jake Muzzin and shot under Campbell's right arm.
"Those were a couple of routine saves," Campbell said. "I give them up and we are down 2-0. That can't happen."
Coach Sheldon Keefe said Campbell should not shoulder the blame alone, though, for Toronto falling behind.
"He's part of spotting them the 2-0 lead early, but it's two guys in alone coming in on him, that's a tough ask for a goaltender," Keefe said. "He's bailed us out a lot, so we've come to expect him to have the ability to save those, and I'm sure he has the expectation to stop them, but he's got five guys playing in front of him, they've got to help him out."
Tavares cut it to 2-1 at 1:51 of the second period with a one-timer off a pass from Mitchell Marner on the power play.
Danault pushed the lead to 3-1 at 13:56 when a pass from Athanasiou hit his right skate and trickled over the goal line. The goal was reviewed for a distinct kicking motion, but it was determined that the puck entered the net in a legal fashion.