With his defensive and pitch calling skills improved, Seminick played an important leadership role during the 1950 "Whiz Kids" championship season. Because he was, at 29, one of the veterans on the squad and was called upon to handle a young Phillie pitching staff, Philadelphia baseball writers nicknamed Seminick "Grandpa Whiz." He enjoyed his best season in 1950, hitting for a .288 batting average with 24 home runs and 68 runs batted in. Seminick broke his ankle late in the season, but continued to play with the injury until the Phillies lost to the New York Yankees in the World Series. Future Hall of Fame pitcher, Robin Roberts said of Seminick,"If you had to pick a guy in the clubhouse who was our leader that year, it would be Andy. He always played hard, and that was his best year by far".