It was Lucroy's second homer of the game and 11th of the season. It was his second career walkoff homer and both were against the Reds. His first was last season off closer Aroldis Chapman and this one was off Sam LeCure.
Francisco Rodriguez (4-3) pitched the ninth to earn the victory.
Cincinnati, which is 0-5 since the All-Star break, brought in LeCure (1-2) to pitch the ninth. His outing lasted just three pitches as Lucroy hit a 1-1 offering for a line drive homer inside the left field foul pole.
Lucroy was batting just .200 (21 for 105) in his last 25 games. His first homer of the game was on a 3-2 pitch from starter Homer Bailey in the sixth.
Milwaukee scored all its runs on solo home runs. Ryan Braun and Aramis Ramirez hit back-to-back homers in the first inning on the one-year anniversary of the start of Braun's 65-game suspension for violating Major League Baseball's drug policies.
Braun hit his 13th homer and Ramirez followed with his 12th homer, and first since June 26.
Cincinnati made it 2-1 in the fourth on Jay Bruce's RBI double that scored Billy Hamilton from third.
Lucroy's first homer extended Milwaukee's lead to 3-1.
Cincinnati tied it 3-3 in the seventh. Milwaukee starter Jimmy Nelson hit Ramon Santiago on the foot with a pitch with the bases loaded and pinch-hitter Chris Heisey hit a sacrifice fly off reliever Brandon Kintzler.
The Reds managed only five hits and Milwaukee had six.
Nelson allowed three runs, four hits and a walk and hit three batters while striking out five in his fourth major league start.
Bailey allowed three runs, four hits and two walks while striking out three in his first start since leaving a game against the Chicago Cubs on July 10 due to right knee soreness.
NOTES: Brewers 2B Scooter Gennett left the game in the fifth inning due to tightness in his right quad. ... Reds SS Zack Cozart left the game after being hit on the right hand by a pitch from Nelson in the seventh inning. X-rays were negative and the team said he was day-to-day. ... The Reds placed RHP Logan Ondrusek on the 15-day DL retroactive to July 13 and recalled RHP Curtis Partch from Triple-A Louisville. ... A brief rain shower caused the Brewers to close roof at Miller Park in the fourth inning. It reopened in the sixth.