Alexey Toropchenko scored, and Jordan Binnington made 28 saves for the Blues (28-22-2), who had won two straight and seven of eight.
“I just think we weren’t skating enough and supporting each other enough and winning battles and being committed to the forecheck,” St. Louis captain Brayden Schenn said. “We know we have to be a whole lot better than we were tonight.
“We relied on our goalie to make saves, and he kept us in the game, but at the end of the day, we just didn't have enough guys, pretty much everyone I should say, that were willing to play the right way tonight.”
McMann put the Maple Leafs ahead 1-0 at 5:33 of the first period. He won a battle for the puck alone the right boards, warded off Schenn as he cut to the net, and scored past Binnington's right pad from the edge of the crease.
“We couldn’t find a lot of energy in our game,” Blues coach Drew Bannister said. “We weren’t moving our feet, and with that we weren’t able to recover any pucks. We lost every race to pucks, which forced us to defend more and we couldn’t get on offense.”
Nylander made it 2-0 at 16:25 when his centering pass deflected off Blues defenseman Matthew Kessel’s stick and fluttered past Binnington’s blocker.
Toropchenko cut it to 2-1 at 5:36 of the second period with a deflection of Nathan Walker's shot below the right hash marks. The goal came after Binnington stopped Noah Gregor on a breakaway at 3:52.
McMann extended the lead to 3-1 at 8:02 of the third period. TJ Brodie's initial shot was blocked by Kessel, but the puck dropped right at the skates of McMann, who picked it up and roofed a shot glove side from the bottom of the left circle.
“It’s nice to see a guy like that who works hard every night, forechecks hard, plays hard, get rewarded," Brodie said. "It’s always nice to see that."
Toronto outshot St. Louis 13-4 in the third period.
“I thought everyone played great,” Brodie said. “We had lots of energy. We got on pucks, got on the forecheck and got the win.”
NOTES: Maple Leafs forward Auston Matthews had his nine-game point streak end (nine goals, six assists). ... Kessel had the secondary assist on Toropchenko's goal. It was his first NHL point in his 19th game. ... Forward Alex Steeves made his season debut for the Maple Leafs after being recalled from Toronto of the American Hockey League on Tuesday. He had one shot and was minus-1 in 10:14 of ice time.