Giancarlo Stanton and Garrett Jones hit two solo homers apiece for Miami, and Khris Davis homered for Milwaukee. The teams combined for seven homers, the most since the pitcher-friendly ballpark opened in 2012.
Reynolds homered in the fourth and again in the fifth, increasing his season total to 11. It was his 20th career multihomer game.
Milwaukee didn't need slugger Ryan Braun, who sat out after a flare-up of the oblique strain that sent him to the disabled list earlier this season. The Brewers hope he can rejoin the lineup by Sunday.
Miami lost despite hitting a season-high four home runs. Stanton's two tape-measure homers increased his season total to 14, most in the NL, and his 47 RBIs lead the majors. It was his 12th career multihomer game, tying Hanley Ramirez's franchise record.
Even so, Miami fell to 5-17 against the Brewers since the start of 2011.
With the retractable roof open on an 83-degree night, the ball was flying from the first inning. Davis' homer carried more than 430 feet into the beer garden, Stanton's homers traveled just as far, and Jones reached the upper deck.
Marco Estrada (4-2) didn't pitch from the stretch until there were two out in the seventh, because all four of Miami's baserunners to that point hit home runs. He gave up five runs and departed after 6 2-3 innings.
Estrada has an ERA of 3.98 despite allowing 16 homers, the most in the majors.
Carlos Gomez had three doubles and a single for the Brewers, who totaled eight extra-base hits. Milwaukee hit for the cycle in the fifth when Jean Segura singled, Scooter Gennett tripled, Gomez doubled and Reynolds homered.
Brewers manager Ron Roenicke made three pitching changes without a glitch. On Thursday at Atlanta, he summoned a reliever when no one was warming up.
Tom Koehler (4-4) went five innings and gave up seven innings, including both homers by Reynolds. Koehler came into the game with an ERA of 0.64 at home, best in the majors.
Miami fell to 19-7 at Marlins Park, still the best home record in baseball.
Notes: Brewers RHP Yovani Gallardo will likely be scratched from his scheduled start Sunday, but he's not expected to go on the disabled list. He threw a bullpen session before the game and said his sprained left ankle was still sore. ... Milwaukee RHP Wily Peralta (4-3, 2.18) is scheduled to start Saturday against Miami RHP Jacob Turner (0-2, 6.59).