Carlos Gomez went 3-for-5 and tacked on a two-run blast for Milwaukee, which snapped a season-high four-game slide.
Lohse (6-1), whose only loss came April 1 at home against the Braves, remained unbeaten over his last nine starts. The right-hander allowed four hits, including an RBI double to Justin Upton in the opening inning. Lohse also fanned eight without walking a batter and moved to 4-0 in six career starts at Turner Field.
Ervin Santana (4-2) yielded nine hits and six runs over seven frames. He walked four batters and dropped his second consecutive start. He's been shelled in both outings, having allowed 10 hits and five runs in five innings Friday at St. Louis.
Upton had three of the five hits for Atlanta.
Gomez singled and Scooter Gennett doubled to right to open the contest. After Ryan Braun popped out, Jonathan Lucroy walked. That set the stage for Reynolds, who clubbed his third career grand slam, a shot to left field on the first pitch of his at-bat.
Freddie Freeman and Upton hit back-to-back doubles with two outs in the bottom of the frame, but the Brewers added to their lead in the fourth. Lohse walked and Gomez followed with his 10th homer of the year, this one to center field.
Lohse was in complete control. He retired 11 straight batters until Upton singled to start the seventh inning. He set down the next six hitters and turned it over to Francisco Rodriguez for the final inning.