Thormalen began his professional career with the class D Chambersburg Maroons and class AA Baltimore Orioles in 1916. When he was 21 years old, he made a one-game appearance for the New York Yankees in 1917 before returning to the minor leagues in 1918. He was once again elevated to the Yankees in 1919. In a six-season major league career, Thormahlen posted a 29–28 record with 148 strikeouts and a 3.33 ERA in 104 appearances, including 64 starts, 27 complete games, four shutouts, three saves, and 565 innings of work. His last game was on May 7, 1925.