Before becoming Tony LaRussa’s right-hand man, Duncan caught for 11 seasons in the big leagues, mostly for the Kansas City/Oakland A’s, but also for the Cleveland Indians and Baltimore Orioles. He came up as an 18-year old with Kansas City in 1964, appearing in 25 games before going to the minors. Duncan spent the next two years there, returning to the A’s in 1967. Mostly a backup, Duncan had a breakout season in 1971, hitting .253 with 15 home runs and making the all-star team in the process. He slugged 19 home runs the next year, helping the A’s reach, and win, the World Series. Duncan went to Cleveland in 1973 and spent two seasons with the Tribe before concluding his career with two years in Baltimore.
Duncan mentions the seventh game of the 1972 World Series as his favorite memory. He was the A’s catcher that game, drawing a walk in four plate appearances, as Oakland defeated Cincinnati to win the championship. Following his playing career Duncan became a well-known pitching coach, serving mainly under Hall of Fame manager Tony LaRussa in Oakland and later with the St. Louis Cardinals. He mentored four Cy Young award winners in this capacity and also won three more World Series (1989, 2006, 2011). He is currently a pitching consultant for the Chicago White Sox.