Ellis had a season-high 37 points and two key assists late, Dirk Nowitzki led a fourth-quarter rally with 14 of his 35 points, and the Mavericks spoiled Howard's best offensive night for Houston with a 123-120 victory over the Rockets on Wednesday night.
The Mavericks settled for a pair of guards in Ellis and Jose Calderon after Howard chose the Rockets in free agency during the summer. Nowitzki wanted Howard as much as anyone else but has raved about Ellis all season.
"We're really just playing off of him," said Nowitzki, who also had a season high in scoring. "He's been aggressive, he's been shooting the ball well, but what's been great is that he has been making plays for others. He's making all of us better."
Howard, playing in front of Dallas fans for the first time since his big decision, made his first 11 shots and scored a season-high 33 points to go with a game-high 11 rebounds after receiving some mild pregame boos.
The Mavericks, who improved to 6-0 at home, trailed by 18 late in the third quarter but were down just one in the final minute when Ellis drove and zipped a pass to Shawn Marion in the corner for the go-ahead 3-pointer with 47 seconds left.
Houston's James Harden had a chance to tie it with 9 seconds left but missed the second of two free throws, keeping Dallas' lead at 121-120. Calderon made two free throws after a Houston foul, and Harden shot an air ball on a 3-point try in the final seconds.
The Rockets had 101 points after three quarters -- 50 of them in the paint -- but set the tone for the fourth by starting it with four straight 3-pointers, all misses.
"In the fourth quarter, we went away from what we were doing the first three quarters," said Howard, who had four alley-oop dunks in the first half. "We've just got to read how teams are playing us. It's growing pains. We have a very young team. We've got to realize what we can do as a team."
Howard, who was 12-of-16 from the field and even made nine of 13 free throws, hit one of two from the line for a 119-114 lead before Nowitzki hit a jumper, then blocked a shot by Howard on the other end.
Ellis assisted on a dunk by Marion, and after Chandler Parsons missed a layup, Ellis found Marion for the go-ahead shot. Ellis finished with eight assists.
"We stayed together as a team," Ellis said. "We got the defensive stops when we needed and came down and made shots on the other end."
Nowitzki passed Reggie Miller for 15th on the NBA scoring list, with the 35-year-old in his 16th season going ahead of Miller on a free throw after a technical foul for defensive 3 seconds called against Howard. Nowitzki now has 25,298 points.
Miller, the second-most prolific 3-point shooter behind Ray Allen, scored 25,279 points in 18 seasons with Indiana from 1987 to 2005.
Brooklyn's Kevin Garnett is 14th at 25,340 points after scoring four in a 95-91 loss to Charlotte on Wednesday night.
"We never could get a handle on them," Rockets coach Kevin McHale said. "We just couldn't get them stopped the whole game. Dirk and Ellis got what they wanted."
The Rockets led by 18 late in the third quarter before the Mavericks rallied in the fourth by shooting 74 percent and outscoring Houston 36-19.
After Dallas got to 104-101, Houston went back up by nine on consecutive 3-pointers from Patrick Beverley and Harden, who had 23 points on 6-of-14 shooting.
The lead was still nine when Calderon and Nowitzki hit 3-pointers around a bucket by Harden, setting up the wild finish.
"It would have been easy to drop our heads and stop playing, but these guys aren't going to do that," Dallas coach Rick Carlisle said.
Parsons had 21 points and a game-high 11 assists, but he attempted just two shots in the second half after scoring 18 and hitting three of four from 3-point range before halftime.
"It's just annoying and frustrating to know we should have won the game," Parsons said. "We had a big lead and then we played completely different in one half than we did in the first half."
Samuel Dalembert, the defensive-minded center Dallas signed after Howard chose Houston, was powerless to stop him most of the night before finally getting a block on a layup try that was just Howard's second miss.
Dalembert led the Mavericks with 10 rebounds and four of their five blocks.
A night after shooting 72 percent in the first half of a blowout win over Boston, Houston was at 68 percent at the break against Dallas and had 36 points in the paint -- 16 of those on dunks -- in taking a 68-61 lead.
Nowitzki got a technical foul in the first half for complaining about a foul on a drive by Harden, who missed the free throw. ... The Rockets shot 75 percent in a 40-point first quarter. Harden, Parsons, Terrence Jones and Omri Casspi combined to go 5-of-6 from 3-point range as Houston took a 40-29 lead.