Mantha, who was named the NHL Second Star for the week ending Oct. 6, gave Detroit (3-1-0) its third lead when his power-play goal made it 3-2 with 4:22 remaining in the second period.
"We're playing great hockey right now," said Mantha, who scored on a one-timer from the top of the right face-off circle. "We're fast, we're playing a lot in our (offensive) zone, in their (defensive) zone, so (we) need to keep it there and our chances are going to come."
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The forward has eight points (six goals, two assists) in four games, including four goals in a 4-3 win against the Dallas Stars on Sunday.
Joel Armia and Max Domi scored, and Carey Price made 30 saves for the Canadiens (1-1-2) in their home opener.
"We're not sharp, and at the end of the day we've got to find that sharpness if we're going to win some hockey games," Canadiens coach Claude Julien said.
Jonathan Bernier made 33 saves for his first win in 12 NHL games in Montreal (1-8-2).
"I waited too long for that one, but obviously it feels nice," said Bernier, who is 2-11-3 in 18 games against the Canadiens.