He batted .248 for their Western Carolina League affiliate, the Salisbury Braves in 1962, and committed seven errors in only 29 games for a .868 fielding percentage.

He made eleven pitching appearances for the California League's Salinas Mets that season, and had a 6.75 earned run average without a decision.

Johnson made his major league debut on July 17, 1968 in the first game of a doubleheader against the Chicago Cubs at Shibe Park.

On October 7, 1969, he was sent by Philadelphia along with Dick Allen and Cookie Rojas to the St. Louis Cardinals in the trade that brought Tim McCarver, Byron Browne, Joe Hoerner and Curt Flood to the Phillies.

With the Giants, Johnson emerged as one of the top relievers in the National League, finishing sixth in Cy Young Award balloting during an era when such an honor was rare for relief pitchers.