Upon joining the Giants, Leemans immediately took over as one of the NFL's most dependable workhorses. The 6-0, 195-pound fullback led the league in rushing as a rookie with 830 yards. He was the only rookie named to the annual all-league team.
At the last regular season game in 1941 the Giants wanted to honor Leemans for his contributions to the team. On December 7 they celebrated "Tuffy Leemans Day," presenting him with a silver tray, a watch, and $1,500 in defense bonds. During the course of the game the stadium announcer had paged Col. William Donovan to answer a call from Washington and had told all servicemen to return to their units, but it was only when the game let out that players and spectators learned of the attack on the naval base at Pearl Harbor earlier that day.
During his outstanding eight-year career, he was named first-or second team all-league every year from 1936 to 1942, by either or both the NFL and a major wire service. A versatile player, at one time or another played fullback or halfback and excelled on defense. At the same time and certainly as a direct result of his play, the Giants were consistently contending for a title berth, and on December 11, 1938 won the 1938 NFL Championship Game against the Green Bay Packers by a score of 23-17. Leemans scored a touchdown on a 6-yard run in the game's first quarter.
Leemans finished his professional career in 1943 with 3,132 yards rushing, 28 receptions for 422 yards, and 2,318 yards passing to his credit. He scored 17 touchdowns rushing, three on receptions, and passed for 25 more. His career ledger also includes punt return and pass interception statistics. His marks become even more significant when it is remembered that the Giants of that era employed a system that saw 2 separate units divide playing time both offensively and defensively.