Pacioretty converted Montreal's first attempt in the shootout after scoring a pair of power-play goals in the second to help the Canadiens rally from a 3-0 deficit. The left wing had nine shots and hit the post twice in his second game since returning from a broken vertebra, the result of a controversial hit by Boston's Zdeno Chara on March 8.
"It's nice to see Max come back," Canadiens coach Jacques Martin said. "He's played two games now and seems to be feeling comfortable."
Scott Gomez drew Montreal even at 3 with 36.5 seconds left in the third when the puck went in off Ottawa captain Daniel Alfredsson. Playing his first preseason game, Gomez centered Pacioretty and prospect Brendan Gallagher on a line that combined for 19 shots.
"It seemed like we were controlling the puck really well every time we had it," Pacioretty said. "It just shows how good Gomer is because most of the time the puck was on his stick down there and we were just trying to create some space for him."
Zack Smith scored goals 1:03 apart in the first period and Milan Michalek got the Senators' third goal midway through the second.
"It doesn't matter if it's preseason or regular season, blowing a three-goal lead isn't good by any means," Smith said. "It's kind of embarrassing, but we did things that put us in the situation for them to come back. They got a couple of power-play goals and that hurt us."
Nathan Lawson stopped all 11 shots he faced after taking over for Montreal starter Carey Price following Michalek's goal. Lawson got an assist with a long pass up the middle that set up Pacioretty's breakaway goal that drew the Canadiens within 3-2 at 17:06 of the second.
Price, who made 15 saves, has allowed seven goals in just over 60 minutes through two starts.