Cahill was a standout player for the great Holy Cross teams of the late 1930s. During his sophomore season he teamed with Bill Osmanski to form one of the Crusaders' most formidable backfields of all-time. That 1937 team would go undefeated, finishing the season with a record of 8-0-2. Cahill would then lead Holy Cross to an 8-1 record during his junior year, and during his senior season (in 1939) the Crusaders went 7-2. Over the course of his three-year varsity career, he helped guide the Crusaders to an overall mark of 23-3-2.
Following his graduation in 1940, Cahill played in the College All-Star Game in which the collegians defeated the New York Giants, completing his first 11 passes in a row. He went on to play one season with the Chicago Cardinals of the National Football League during the 1943 season, connecting on 50 of 109 passes for 608 yards and three touchdowns in 10 games played (including two starts).