The Mets averted being swept in the three-game series and won for just the seventh time in 19 games. Meanwhile, the Braves' four-game winning streak ended.
Nimmo, who left after four innings in Saturday's 4-1 loss due to a tight right intercostal muscle, entered as a pinch runner for DJ Stewart in the seventh inning on Sunday.
Atlanta snapped a 2-2 tie in the eighth, when Marcell Ozuna delivered his second RBI single of the night, and the Mets went down in order in the bottom of the inning before coming back in the ninth against A.J. Minter (5-3).
Jeff McNeil legged out a bunt single and went to second on a sacrifice bunt by Tomas Nido. Nimmo then hit a 3-2 pitch beyond the fence in right-center field for his second career walk-off homer.
Check out the game summary here.
Cubs 5, Pirates 4 (10 innings)
Patrick Wisdom hit a solo homer during a three-run 10th inning as visiting Chicago beat Pittsburgh in the rubber match of a three-game series.
Cubs reliever Adbert Alzolay allowed Connor Joe's two-run homer in the bottom of the 10th before securing his fourth save. Hector Neris (3-0) earned the win after escaping a bases-loaded, one-out jam in the ninth inning. Wisdom had three hits and Christopher Morel had two, including a two-run homer.
Oneil Cruz had a solo shot for the Pirates, who went 3-for-10 with runners in scoring position and fell to 9-21 in their past 30 games.
Royals 4, Angels 2
Seth Lugo struck out a career-high 12, leading Kansas City past Los Angeles in Anaheim, Calif.
Lugo (6-1) went a season-best eight innings, allowing one run on five hits. He did not walk a batter. James McArthur gave up a solo home run to Willie Calhoun in the ninth but earned his ninth save.
The Royals scored all four of their runs in the fourth inning against Angels starter Patrick Sandoval (2-6), who gave up 11 hits and struck out three in six innings.
Diamondbacks 9, Orioles 2
Zac Gallen pitched six strong innings and Arizona finally got its offense going to avoid a three-game sweep, defeating host Baltimore.
Jake McCarthy hit his first home run of the season - a two-run shot - during Arizona's six-run sixth inning. Ketel Marte drove in three runs for the game. Gallen (5-2) allowed two runs and four hits.
Adley Rutschman homered for the Orioles, who lost for only the second time in their past nine games. Baltimore starter Dean Kremer (3-3) gave up six runs (three earned) on four hits and four walks while fanning 10 in 5 2/3 innings.
