Johnsson assisted on Andreas Borgman’s winner as well as Martin Marincin’s goal for Toronto (4-2-0). Connor Carrick also scored in the Maple Leafs’ three-goal second period. Curtis McElhinney made 29 saves for the win.
"The most important things are to play well in the system and work as hard as I can," said Johnsson, a 22-year-old left-winger who spent all of last season with the American Hockey League’s Toronto Marlies.
Jonathan Drouin and Andrew Shaw, on the power play, built a 2-0 lead for Montreal (0-6-0) in the first period. Carey Price stopped 24 shots in net.
The game was held at Quebec City’s Videotron Centre, with 14,452 fans in attendance.
The Maple Leafs won even though they had left all their big guns in Toronto and were facing a Canadiens’ lineup stacked with veterans including Drouin, Shaw, Max Pacioretty, Brendan Gallagher, Alex Galchenyuk, Shea Weber and Price.
"The lineups can be a little misleading," said McElhinney, "I give credit to those kids for coming out and working hard."
Drouin and Shaw, who returned from injuries, were both playing in their second games of the pre-season and got three and four shots respectively on McElhinney.
The Marincin and Borgman goals were scored 99 seconds apart and gave the Maple Leafs a 3-2 lead heading into the second intermission.
The Canadiens and Maple Leafs each have two games left in the pre-season. Montreal hosts the Florida Panthers on Friday and the Ottawa Senators on Saturday.
Toronto plays a home-and-home series against the Red Wings — Friday in Detroit and Saturday in Toronto.