On June 16, 2015, Collins won his 340th game as Mets manager, passing Gil Hodges for the third-most in franchise history. On September 26, 2015, the Mets defeated the Cincinnati Reds 10–2 to clinch the National League East. It was the first time Collins ever clinched a playoff berth as a manager. On October 15, 2015, the Mets defeated the Los Angeles Dodgers to advance to Collins's first-ever National League Championship Series appearance. On October 21, 2015, the Mets defeated the Chicago Cubs to advance to the 2015 World Series versus the Kansas City Royals. On November 2, the Royals won game 5 in Citi Field to defeat the Mets and win the World Series, 4 games to 1. Collins made the decision to leave starter Matt Harvey in the game for the ninth inning with the Mets holding a 2-0 lead. Harvey gave up 2 earned runs in the inning to allow the Royals to tie the game, leading to questions about Collins's strategy.