After retiring, Dellinger was an assistant coach to Bill Bowerman for the Oregon Ducks track and field team. He became head coach after Bowerman's retirement in 1972. In 25 years as coach, Dellinger won five NCAA titles, coached 108 All Americans, and had a 134-29 meet record. He was instrumental in the development and coaching of Oregon and American great distance star Steve Prefontaine in conjunction with Bowerman, and their experience was made into a 1997 film Prefontaine, in which Bill Dellinger was played by Ed O'Neill. Dellinger also coached many post-collegians including Olympians Mary Decker, Alberto Salazar, Matt Centrowitz, Don Clary, and many others.
Dellinger joined his mentor Bill Bowerman as an inductee in the National Track and Field Hall of Fame in 2001.