A native of Superior, Wisconsin, Leemans played college football for Oregon's freshman team in 1932 and for George Washington from 1933 to 1935. He was drafted by the New York Giants in the second round of the 1936 NFL Draft and played eight years for the Giants from 1936 to 1943. He led the National Football League as a rookie with 830 rushing yards and was selected as a first-team All-Pro in 1936 and 1939. He was also selected to play in the Pro Bowl in 1938 and 1941 and helped lead the Giants to the 1938 NFL Championship and the 1939 and 1941 NFL Championship Games.