It was his 12th goal of the season, and he snapped a 23-game goal drought. His previous snipe came on March 13 against the Calgary Flames.
Tyler Toffoli assisted on Petry's goal.
Centreman Nick Suzuki thought he tied things up two minutes later on the power play, but the goal was disallowed because Corey Perry came into contact with goaltender Filip Gustavsson.
But, the Canadiens wouldn't be denied.
Toffoli capitalized on another power play with 5:35 remaining in regulation to score his 27th goal of the season.
The 29-year-old right-winger converted a pefect feed from Suzuki, blasting a one-timer past Gustavsson to make it 2-2.