After retiring as a player, Seminick worked for the Philadelphia organization for the rest of his life. He was a coach with the Phillies (1957–58 and 1967–69) and manager of 11 minor-league affiliates (1959–66 and 1970–73). After that, he served as a scout and as a roving minor-league instructor for the Phillies (from 1974 into the mid-1980s). Notably, ninety of the players he managed or coached eventually played in the major leagues, including Mike Schmidt, Ferguson Jenkins, Greg Luzinski and Bob Boone. Seminick helped to convert Boone from a third baseman to one of the best defensive catchers in baseball.