Kyle Lohse pitched 6 2-3 strong innings to hand the Reds their sixth straight loss since the All-Star break.
Lohse (11-4) walked two, struck out three and allowed four hits, the last to pinch-hitter Donald Lutz who doubled. Reliever Zach Duke retired Billy Hamilton on a chopper back to the mound to end the seventh.
Reynolds connected on a 1-1 pitch from Reds starter Mike Leake (7-9) for his 15th home run of the season. Rickie Weeks was on with a double to start the sixth.
Reynolds' second home run came off reliever Jumbo Diaz in the eighth.
The Reds trailed the NL Central-leading Brewers by 1 1/2 games before the All-Star break, but the loss dropped them 5 1-2 games behind.
Lohse and Leake were locked in a scoreless until the fifth inning. The Reds pushed across a run on Hamilton' sacrifice fly, but the Brewers tied it with a run scoring triple from Carlos Gomez and then took the lead when Gomez scored on Ryan Braun's infield single.
Second baseman Skip Schumaker ranged far to his left and snared Braun's grounder deep in the hole at second. Braun beat the throw in a close play as Gomez raced home.
Right fielder Jay Bruce made a diving lunge for Gomez' sinking liner and missed. Lohse, on with a single, barely beat the relay throw home.
Leake allowed 11 hits in six innings as the Reds lost a season-high nine road games.
Brewers reliever Jeremy Jeffress, whose contract was selected from Triple-A Nashville, made his debut for Milwaukee with a 1-2-3 ninth inning.
NOTES: Brewers 3B Aramis Ramirez played his 2,000th major league game in the city where it all started. On May 26, 1998, he debuted with the Pittsburgh Pirates at Milwaukee County Stadium. . Reds SS Zack Cozart missed the game. He left Tuesday's game after he was hit by a pitch on the first two fingers on his right hand. Manager Bryan Price said he could pinch hit. . Brewers 2B Scooter Gennett left Thursday's game in the fifth inning due to tightness in his right quad. He is day-to-day. . Alfredo Simon (12-4) starts Friday when the Reds take on the Washington Nationals in Cincinnati. Thursday, the Brewers Matt Garza (6-7) faces the New York Mets Dillon Gee (4-2). Garza looks to bounce back from his shortest start of the season when he exited with one out in the first inning of an 8-3 road loss to the Nationals.