Diffley attended Albertus Magnus High School of Rockland, New York where he played on the soccer team from 1981 to 1984.
During Diffley's freshman year, American went to the NCAA title game where the Eagles fell to the University of California, Los Angeles in eight overtimes.
In 1989, Diffley began his professional career with the Albany Capitals of the American Soccer League (ASL).
That summer, Diffley moved to the Penn-Jersey Spirit of the American Professional Soccer League (APSL).
When the Wizards released him, Diffley became a full-time coach but continued to play for the Staten Island Vipers in 1998 and 1999.
In 1997, he moved to St. John's University where he served as an assistant coach with the men's soccer team while working on his master's degree in business administration.
On January 1, 2004, Diffley returned to St. John's when the school hired him as its Senior Associate Athletic Director for Administration.