The Marlins dugout erupted in the ninth inning when McGehee connected for just his fourth home run of the season, a two-out, two-run blast that sent Miami to a 6-3 win over the Brewers on Tuesday night.
McGehee, who played for Milwaukee from 2009-11, is among the National League leaders in hits. He get the best of Brewers closer Francisco Rodriguez (4-5) and snapped a 3-3 tie.
"It's probably just out of sheer surprise that I drove the ball. It is more the shock factor when I actually hit one out now," McGehee said of his teammates' reaction.
Marcell Ozuna followed with another homer to complete the back-to-back blasts, and send the fading Brewers to their 13th loss in 14 games.
McGehee hit a changeup from Rodriguez, who has given up a career-high 14 home runs this season.
"You can't say enough about (McGehee)," Miami manager Mike Redmond said. "I guess we have talked about him not hitting a lot of home runs, so if there was ever a time to hit a big home run, today was it. That was probably as good of a feeling in the dugout as we've had all year. Our guys were fired up."
The Marlins broke through in the third inning against Milwaukee starter Matt Garza. Christian Yelich led off with a single, stole second and scored on Donovan Solano's base hit. Ozuna, who reached base five times, added a run-scoring single in the inning.
"I'm just getting patient at the plate," Ozuna said.
Garza lasted just four innings in his second start after missing nearly four weeks with an oblique strain. He gave up six hits, two runs -- one earned -- and one walk. He struck out seven.
The Brewers tied it in the bottom of the third off Tom Koehler, who gave up six hits and two runs. He walked two and struck out three in five innings.
CAN'T GO HOME:
The Brewers loaded the bases with no outs against Morris in the eighth inning but failed to score. Back-to-back base runners were forced out at the plate before Morris retired Gennett on a fly ball to right. Gennett flipped the bat in disgust as he jogged toward first base.
ONE THAT GOT AWAY
A botched pickoff by Garza in the third inning set up the Marlins' second run. As the slow-footed McGehee broke for second, Garza spiked a throw into the dirt that skipped into the outfield grass. McGehee later scored from second on Ozuna's single, leading to an unearned run.
Brewers: Struggling OF Ryan Braun wasn't in the starting lineup due to a sore thumb and hand. "It's part of the reason he's been swinging the bat the way he has," manager Ron Roenicke said. Braun entered the game as a pinch-hitter.
Marlins: Henderson Alvarez, who left his last start on Sept. 1 with an oblique strain, threw a light bullpen session on Tuesday and is expected to return to the rotation on Friday at Philadelphia. "I think we are all fortunate that this wasn't a year-ender for him," Redmond said.
Brewers: Wily Peralta (15-10, 3.84 ERA) has lost his last three starts while posting a 10.29 ERA. Peralta lost his only other start against the Marlins this season despite giving up just one run in six innings at Miami on May 24.
Marlins: Jarred Cosart (4-1, 1.99) has never faced the Brewers. He allowed two runs and five hits in 7 2/3 innings in his last start on Friday against Atlanta.