Phelps began his coaching career in 1963 as a graduate assistant at Rider College (now Rider University), where he had played basketball. After a move to St. Gabriel's High School in Hazleton, Pennsylvania, he obtained his first full assistant job in 1966 at the University of Pennsylvania.

His first head coaching job came in the 1970–1971 season at Fordham University, where he coached Charlie Yelverton and P.J. Carlesimo. After leading the Rams to a 26-3 record, a Number 9 national ranking and an at large bid to the NCAA tournament, he was named head coach at the University of Notre Dame.

During his 20 seasons at Notre Dame (1971–91), his teams went 393–197, with 14 seasons of 20 wins or more. In 1978, Notre Dame made its only Final Four to date. His most-remembered game was on January 19, 1974, when the Fighting Irish scored the last 12 points of the game to defeat top-ranked UCLA, led by John Wooden, 71–70, ending the Bruins' record 88-game winning streak. He shares the NCAA record for most upsets over a #1 team at seven.

Phelps got his first broadcasting experience when he served as commentator for ABC Sports' basketball coverage at the 1984 Summer Olympics in Los Angeles.

In 1992, he began his broadcasting career as he announced color commentary for that year's NCAA tournament for CBS. He joined ESPN the next season and worked for them until 2014 as a college basketball studio and game analyst.

Phelps announced during the 4/7/14 broadcast of College GameDay that he is leaving ESPN.

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