On May 19, 1971 — In its inaugural season, Riverfront Stadium’s first-ever inside-the-park home run is hit by Roberto Clemente, a smash off Gary Nolan. The ball is hit high off the left-field wall over the reach of Bernie Carbo. The ball bounces high and back toward the infield past Buddy Bradford in center and by the time he is able to retrieve it on the rain-soaked turf, Clemente is rounding third. Clemente adds a triple that drives in a run and comes to score himself, then singles in another run in the 9th inning in a 6 – 1 decision over Cincinnati.

Source:
Baseball Reference May 19