A left-hander, Shantz began his career as a starting pitcher, but about halfway through he converted to a competent relief pitcher. In 1951 he added the knuckleball to his repertoire. Standing only 5 ft 6 in (1.68 m), Shantz had a career record of 119 games won, 99 games lost, and an earned run average of 3.38.
Shantz enjoyed his best season in 1952 when he led the American League in wins (24) and won the MVP Award. In the process, he led the A's to a fourth-place finish. This was their last winning season in Philadelphia. The 1952 A's had some good players, including batting champion Ferris Fain, shortstop Eddie Joost, home run hitter Gus Zernial, and fleet center fielder Dave Philley. In a September 1952 game, Shantz's left wrist was broken after it was hit by a pitch thrown by Walt Masterson. The following season, Shantz injured his shoulder in a game against the Boston Red Sox. Shantz saw only limited action for the rest of 1953 and pitched only eight innings in 1954. After the 1954 season was complete, the Athletics were sold and moved to Kansas City for the 1955 season.