Dumatrait exited after just 12 pitches. He did not retire any of the five batters he faced, allowing four earned runs on five hits including the three home runs.
"He didn't have command of any of his pitches," said Reds interim manager Pete Mackanin. "The Brewers let him know about it. They came out swinging. I had to get him out of there. You don't want to see anyone get embarrassed out there."
The win, coupled with Chicago's loss at Pittsburgh, moved the Brewers into sole possession of first place in the NL Central for the first time since Aug. 16.
Edwin Encarnacion hit a three-run home run, his 12th of the season, in the loss.
The seven combined homers hit Sunday are tied for second-most in Great American Ball Park history.
A historic moment occurred in the first inning when Rickie Weeks, J.J Hardy and Ryan Braun hit consecutive homers off Dumatrait.
"We set the tone early," said Weeks. "It was pretty special. I've never been a part of [back-to-back-to-back] home runs."
The last time a team accomplished the feat was on April 13, 1987, when San Diego's Marvell Wynne, Hall of Famer Tony Gwynn and John Kruk went deep off the Giants' Roger Mason.
The only other time a club had begun a game with three straight homers was in 2003 when Rafael Furcal, Mark DeRosa and Gary Sheffield did it off the Reds' Jeff Austin.
"All fastballs," said Reds catcher Javier Valentin of the three homers. "[Dumatrait] has good stuff. He just has to locate his pitches. I feel sorry for him. He's still learning. I've never seen [three homers] to lead off a game."
Dumatrait (0-4) was unavailable for comment afterwards.
"That might have been my quickest hook ever," said Mackanin. "There were plenty of other pitchers to see."
Tom Shearn, making his first relief appearance of the season, allowed three earned runs on four hits in two innings.
The Reds spotted Milwaukee starter Ben Sheets a four-run lead. Not the desired recipe for success.
Sheets (12-4) was 2-0 with a 2.37 ERA in three previous starts at homer-friendly Great American Ball Park. He didn't fare quite so well on Sunday, allowing five earned runs on six hits in six innings, but it was good enough to earn him the victory.
"I got a little tired toward the end," Sheets said.
Braun and Weeks each homered twice for Milwaukee. Braun's 30 homers are a Brewers rookie record.
Ken Griffey Jr. drove home the Reds' first run in the first inning with a sacrifice fly, giving him 1,700 career RBIs.
Brandon Phillips, who's trying to become the third Reds player and first since Barry Larkin in 1996 to have 30 home runs and 30 steals in a season, stole his 28th and 29th bases Sunday. He currently has 28 homers.
Ryan Hanigan collected a pinch-hit double off Sheets in the fifth inning in his first career at-bat. He scored moments later from third on a wild pitch to make the score 8-2.
"That's a pretty good trophy," said Mackanin of Hanigan's hit. "I'd rather get my hit off a guy like Sheets than Joe Bag O' Donuts."
Encarnacion's three-run home run, a 416-foot blast off the batter's eye in center, made the score 8-5 in the sixth.
Johnny Estrada hit a two-run homer in the eighth to put Milwaukee ahead, 10-5.
Mackanin said a decision on Dumatrait's future could come this week.
"We're going to discuss it," Mackanin said. "I don't think he's done. He has a future here. It's a learning process. We'll discuss what to do for the rest of this year. He has the stuff to be successful."