Byrd hit the first pitch he saw from Jake Dunning in the eighth inning into the left-field seats to break the game open and give the Mets a back-to-back wins over the slumping World Series champs. The Giants lost for the seventh time in eight tries.
Anthony Recker added a two-run home run for the Mets and Andrew Brown also drove in a pair of runs. Daniel Murphy's sacrifice fly in the eighth put New York ahead 6-5 and set the stage for Byrd's insurance slam.
Scott Rice (4-5) picked up the win in relief of Dillon Gee.