Midway through the fourth quarter, Thomas broke out in transition and finished a layup in traffic for his 20th point of the night. Thomas shuffled ahead of Havlicek, who set the record during the 1971-72 season.
Thomas, who finished with 29 points, has now scored 20 or more points in 52 of his 53 games this season and could challenge Havlicek's franchise record of 76 total games with 20 or more points, set during the 1970-71 season.
Celtics coach Brad Stevens marveled at Thomas' consistency during the streak.
"I think consistency is the mark of really great players. I don't think there's any question about it," Stevens said after Thursday's game. "There's a lot of people that can [score 20 points] once or twice but to do it every night, week after week after week, is pretty impressive."
Asked about passing Havlicek, Thomas downplayed the feat, telling reporters, "I'm trying to win a championship and break those type of records."
Thomas is only the 17th player in NBA history to score 20 or more points in 40 consecutive games in one season, according to Elias Sports Bureau.