The Reds had just four hits off Brewers starter Kyle Lohse, and five overall, and fell to 0-6 since the All-Star break -- their first six-game losing streak in more than three years.
Lohse (11-4) allowed one run, two walks and struck out three for NL Central- leading Milwaukee and Reynolds had three hits and three RBI.
Mike Leake (7-9) gave up four runs on 11 hits in six innings for the Reds to lose his third straight start.
Cincinnati has scored 12 runs in its first six-game slide since May 18-23, 2011.
The Reds scored first on Billy Hamilton's sacrifice fly in the top of the fifth but the Brewers took the lead with two runs in the bottom of the inning.
Lohse sparked the rally with a one-out single and scored when Carlos Gomez tripled the other way past a diving Jay Bruce in right field. Ryan Braun beat out an infield single to second base to drive in the second run.
The Brewers pushed their lead to 4-1 on Reynolds' first homer of the game, a two-run blast to left in the sixth inning. Reynolds hit his 16th homer of the season with two outs in the eighth off Jumbo Diaz.