The defending champion Mavericks reached the midpoint of their regular season schedule after never trailing against Boston.
Nowitzki, who also had a season-high 16 rebounds, scored all of Dallas’ points in a 10-2 run midway through the second quarter that pushed the Mavericks ahead by 14. They led by double digits the rest of the way.
Already without suspended Rajon Rondo, absent Kevin Garnett and injured Brandon Bass, the Celtics lost starters Jermaine O’Neal (sprained left wrist) and Chris Wilcox (right adductor strain) in the third quarter.
Boston has lost four in a row, and six of seven.
Nowitzki moved to No. 20 on the NBA scoring list with seven points in the first quarter. That pushed him one past Hall of Famer Robert Parish (23,334), who played 14 of his 21 NBA seasons for the Celtics.
After finishing with his 26 points in 31 minutes, Nowitzki pushed his career total to 23,354 points. All of those have come the past 14 seasons, all with Dallas. Nowitzki is set to play in his 11th consecutive All-Star game this weekend.
At No. 19 on the scoring list is Garnett (23,719), who missed his second consecutive game for personal reasons.
Jason Terry added 16 points with four 3-pointers for Dallas, and Shawn Marion had 11 points.
Jason Kidd, who began the game tied with Michael Jordan for second on the NBA career list for steals, had three against the Celtics to push his career total to 2,517.
Paul Pierce led the Celtics with 20 points. Ray Allen added 15 and Avery Bradley 12.
Rondo was suspended two games without pay by the NBA after throwing a ball at an official during Sunday’s loss at Detroit. He received two technical fouls and ejected from a game for the first time in his professional career.
Bass, a former Mavericks player who is one of Boston’s top reserves, was inactive for the fifth consecutive game because of a sore left knee.
Both teams were playing the second night of back-to-back games.
The Mavs were coming off a 104-97 loss at New York that ended their six-game winning streak even while Nowitzki scored a season-best 34 points.
Dallas was already up 44-34 at halftime against the Celtics when Wilcox picked up his fourth foul on Boston’s first offensive possession of the second half. It wasn’t clear when or how he got hurt, but he was out before the end of the third quarter.
Kidd then hit a 3-pointer before Nowitzki’s step-back 13-footer made it 49-34.
The Mavs built as much as a 24-point lead, though Boston cut that to 15 before Marion’s left-handed tip-in of a Lamar Odom miss at the end of the third quarter.
NOTES: Dallas swept the season series against the Celtics for the second season in a row. … Both teams play their final games before the All-Star break on Wednesday. The Mavericks are home against the Lakers. Boston plays at Oklahoma City.