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.