At the age of 23, Seminick made his major league debut on September 14, 1943. By 1945, the Phillies were using Seminick in a platoon system alongside veteran catcher Gus Mancuso. While he could hit for power, his defensive skills were below average, as he led the National League in errors in 1946, 1948 and 1949. Phillies manager Eddie Sawyer assigned Phillies coach and former catcher Cy Perkins to tutor him, which helped improve his defensive skills. He was voted by baseball fans to be the starting catcher for the National League in the All-Star Game, mostly for his reputation as a hitter.