Pacific Coast League Caster signed with the
San Francisco Seals of the
Pacific Coast League in 1929. He was farmed out to the
San Bernardino Padres in the
California State League for the 1929 season.
Philadelphia Athletics In September 1934, Caster made his major league debut with the
Philadelphia Athletics. He compiled a 3–2 record for the A's in 1934 and returned to the team in 1935. He appeared in 25 games for the 1935 A's, all but one as a relief pitcher. He finished the season with a 1–4 record and 6.25 ERA. Caster was optioned by the Athletics to the Portland Beavers for the 1936 season, which proved to be the best of Caster's career. In 44 games for Portland, he compiled a 25–13 record and a 2.79 ERA – a career high in wins and a career low in ERA. In a front page profile,
The Sporting News credited Caster's turnaround to overcoming his lack of control and adding a slow knuckleball to his blazing fastball and sharp-breaking curve. After a strong showing in Portland in 1936, Caster was recalled by the A's for the 1937 season. In 1939, Caster pitched on opening day for the Athletics against the Washington Senators in Washington, D.C. President
Franklin Roosevelt attended the game, in which Caster pitched a shutout; Caster later described the opening day shutout in front of the President as one of the greatest thrills of his baseball career.
St. Louis Browns On November 16, 1940, Caster was selected off waivers by the
St. Louis Browns from Philadelphia. In 1941,
Luke Sewell converted Caster into a relief pitcher, and he had the best seasons of his major league career as the Browns' lead reliever from 1942 to 1944. As a reliever, his
Adjusted ERA+ ratings skyrocketed to 133, 159, and 150 between 1942 and 1944. Caster won five straight games for Detroit before finally losing a game. The 1945 Tigers went on to win the
American League pennant and defeated the
Chicago Cubs in the
World Series. Caster appeared in 22 games for the 1945 Tigers, all in relief, compiling a 5–1 record and 3.86 ERA in 51 innings. He also pitched of an inning of no-hit baseball in the World Series. In a lengthy feature story on Caster at the end of the regular season,
The Sporting News credited Caster's contributions with this headline: "Caster Helps to Keep Tiger Wagon Rolling With Smooth Relief Hurling: 36-Year Fireman Cops Five Straight in Smothering Blazes." Caster returned to the Tigers in 1946 and compiled a 2–1 record (5.66 ERA) in 26 relief appearances. He pitched his final major league game on September 16, 1946, at age 39.
Return to minor leagues On December 12, 1946, Caster was released by the Tigers. He played for San Diego and Hollywood in the Pacific Coast League during the 1947 season. In 1948, he was a player/manager for the
Riverside Rubes in the
Sunset League. He attempted a comeback five years later, serving as a player-coach and appearing in eight games for the
Edmonton Eskimos of the
Western International League in 1953. ==Personal life and death==