Pacioretty assisted on Andrei Kostitsyn’s goal 10:26 into the second and scored 2:12 later to make it 3-1. He added his second goal of the game in the third.
Yannick Weber scored a power-play goal late in the first period to tie it at 1. Michael Cammalleri also scored for Montreal, which got its first win in six games at the Bell Centre (1-3-2).
The Canadiens, who are 2-5-2 overall, had not gone this deep into a season before claiming their second win since 1941-42, when they got off to a 1-10-1 start.
Jaromir Jagr scored his third goal in two games for Philadelphia, which played its first game without injured captain Chris Pronger.
Ilya Bryzgalov made 23 saves.
Canadiens assistant coach Perry Pearn was fired before the game. GM Pierre Gauthier told his players the news in the team’s dressing room a few hours before the pregame warmup.