Willenborg's amateur career was highlighted by a USTA National Clay Court Championship and a top 5 national junior ranking. He was selected to the prestigious Junior Davis squad and competed at Roland Garros, Wimbledon and US Open.

In 1982 Willenborg accepted a full scholarship to UCLA, where he helped the Bruins win ITA National Indoor and NCAA Championships in 1984. He completed his college career at the University of Miami as a top 25 singles player.

Following his professional career, Willenborg joined the staff at the University of Miami in 1989, where he served as an assistant men's coach for six seasons. During that time, Miami was consistently a top 10 team and Willenborg coached two top-ranked singles players. In 1996, he took the position as assistant tennis coach at Pepperdine University, where he won two consecutive West Coast Conference Championships and was promoted to interim head coach for the 1997 season. The team completed the season with a 22-7 record, including a win over top-ranked UCLA in the season-finale, and a top 10 national ranking.

Willenborg also served as the assistant men's and women's coach at perennial Division II power Barry University. The men's team was an NCAA finalist both seasons of his stay, while the women earned their first ever Sunshine State Conference Championship and a third-place finish - their highest ever - at the NCAA Championships.

A 1987 graduate of Miami with a degree in physical education, Willenborg earned a master's in sports management from the U.S. Sports Academy in 1997 and a master's in computer science from Barry in 2000. He resides in Fork, Maryland, with his wife Amity and children Russell (8), Madeline (6) and Asher (5).