The Nationals took a 4-0 lead into the fifth but the Marlins came back to down the Nationals for the second straight game.
Coghlan doubled to kick off Florida's key fifth-inning and moved to third on a fly out by Infante. A double by Hanley Ramirez drove in Coghlan, and after Gaby Sanchez and Logan Morrison drew walks off Livan Hernandez, Buck's triple to right tied the score.
Brian Sanches (1-0) picked up the win with one inning of relief. The loss went to Chad Gaudin (0-1), who gave up the two sixth-inning runs on three hits.
Leo Nunez struck out two in the ninth to collect his first save.