Sadecki, a southpaw, made his big league debut with the St. Louis Cardinals in 1960 at the age of 19 and would win 9 games that season. In 1964, he would win 20 games for a Cardinals team that overcame the Philadelphia Phillies in the last two weeks of the season to win the NL pennant and eventually win the World Series against the New York Yankees. Sadecki was the winning pitcher in Game 1 besting Yankees ace Whitey Ford.
Things would go south for Sadecki in 1965 as he fell to 6-15 wih a 5.21 ERA. Midway through the 1966 season, the Cardinals dealt Sadecki to the San Francisco Giants for future Hall of Famer Orlando Cepeda in what proved to be an unpopular trade. In 1968, Sadecki would lose 18 games which would tie him for the NL lead along with Claude Osteen of the Los Angeles Dodgers. Despite going 12-18, Sadecki would attain career highs in strikeouts (206), complete games (13) and shutouts (6) along with a respectable 2.91 ERA. Such things were possible in 1968, the Year of the Pitcher.
Sadecki would spend one more season with the Giants before being traded to the New York Mets prior to the 1970 season. He would largely pitch out of the bullpen and would appear in one more World Series when the Mets won the NL pennant in 1973. Sadecki briefly returned to the Cardinals in 1975 and also pitched with the Atlanta Braves, Kansas City Royals, Milwaukee Brewers before finishing his career back with the Mets in 1977.