He was the head coach of the University of Detroit football team for six seasons between 1917 and 1924 and led the program to national prominence.

His coaching career was interrupted by one year of military service during World War I and by his retirement after the 1922 season.

After retiring from football, Duffy practiced law and served for a time as Michigan's boxing commissioner.

[3] In 1916, Duffy accepted a position as the football coach at Eastern High School in Detroit.

[1][4] In August 1917, following the sudden resignation of Gil Dobie, Duffy was hired as the football coach at the University of Detroit.

After the 1922 season, Duffy retired as the school's head football coach in order to begin a law practice.

[14] During Duffy's time as the school's head coach, the team was "ranked among the country's gridiron giants.

[1][4] At the time, the Detroit Free Press described him as "a chunky grey-eyed nervous little man 'with the map of Ireland stamped upon his face.