Originally signed by the St. Louis Cardinals as a free agent, Hamilton debuted in MLB as a starting pitcher for the Phillies in 1962 and posted a 9–12 record with an earned run average of 5.09.

Pitching for the Mets on May 4, 1966, Hamilton tossed a one-hitter against the Cardinals in St. Louis; the one hit being a bunt single by opposing pitcher Ray Sadecki.

On August 18, 1967, the Angels were playing the Boston Red Sox in a game that would have important implications for the American League (AL) pennant race.

Conigliaro made a promising-but-brief comeback in 1969–1970, until his vision problems returned, which eventually forced his early retirement from baseball in 1975 at age 30.

When Jack retired, he and his wife Jan got into the restaurant business opening eateries in various cities including his hometown of Morning Sun, IA.