John Diffley was a four-year standout for the Eagles men's soccer program. Diffley was twice a member of the Colonial Athletic Association First Team and was named the American University Male Scholar Athlete of the Year in 1988. He finished his AU career tied for ninth all-time in career assists. In 1988, Diffley was a member of the U.S. National Team. He made 26 appearances with the National Team, and was a member of the 1988 U.S. Olympic Team.
Diffley's success continued after college. He captained Tampa Bay and Kansas City of Major League Soccer and also played abroad in Holland. After his playing career, Diffley moved on to the coaching ranks, spending two years as the head coach of Dominican College. He then moved on to become an assistant coach at St. John's University from 1997-2001, helping the Red Storm to the 1997 Big East regular season title and the 1998 Big East Tournament Championship.