Muncy also singled, all after Mookie Betts had led off the game with his second homer of the season for the Dodgers, who snapped a three-game losing streak.
Reds starter Graham Ashcraft allowed two runs on six hits in six innings with seven strikeouts and three walks.
Ryan Mountcastle drilled a two-run homer in the first inning and Austin Hays added a solo shot in the sixth.
Alvarez drove in two runs to boost his major-league-leading RBI total to 16, plating a pair in the second inning with a single.
Jake Cave hit a solo homer and double while Nick Castellanos added three hits and three RBIs.