[1][2][a] Standing 6 feet 1 inch (1.85 m) and weighing about 200 pounds (90 kg), Bithorn was a hard-throwing pitcher who commanded attention when he began his delivery with a distinctive windup, raising his long left leg high in the air and throwing a blazing fastball toward home plate.

The Puerto Ricans faced the Richmond BBC, a squad composed entirely of continental American players, including slugging first baseman and future Hall of Famer Johnny Mize, as the 16-year-old Bithorn led his team to a 10–1 victory over the visiting club.

[4] Bithorn was signed by the Chicago Cubs in September 1941 and debuted in the Major Leagues on April 15, 1942, making history as the first Puerto Rican to play in organized baseball.

[5][a] Bithorn won nine games and lost 14 in his first season,[5] but he rebounded in 1943 by going 18–12 with an earned run average of 2.60 and completing 19 of his 30 starts,[5] leading the league in shutouts with seven, establishing a record for Puerto Rican pitchers that still stands to this day.

In a four-season career, Bithorn posted a 34–31 record with a 3.16 ERA in 105 pitching appearances (53 as a starter), including 30 complete games, eight shutouts and five saves, striking out 185 batters while walking 171 in 5092⁄3 innings of work.

[6][9] Bithorn's achievement of making it to the majors remained a source of pride in Puerto Rico, and he was honored in 1962 when the biggest ballpark on the island was built and named after him.