At the Polo Grounds‚ Boston pitcher Pat Flaherty clubs the first grand slam by a National League pitcher when he connects in the 2nd inning off Hooks Wiltse of the Giants. Boston wins‚ 7 – 5.

On May 24, 1907 — At the Polo Grounds‚ Boston pitcher Pat Flaherty clubs the first grand slam by a National League pitcher when he connects in the 2nd inning off Hooks Wiltse of the Giants. Boston wins‚ 7 – 5.

Source:
Baseball Reference May 24