Melchionni was a wispy guard from Bishop Eustace Prep (Pennsauken, New Jersey) who was a collegiate star in the mid-1960s at Villanova University and was the Most Valuable Player in the 1966 NIT, when Villanova finished in third place in the tournament. Upon graduation from Villanova, Melchionni joined the famous Phillips 66ers AAU Elite team, where he starred for a season before returning to his hometown to join the Philadelphia 76ers.
Melchionni played in three ABA All-Star Games, and was named to the All-ABA First Team in 1972. Melchionni led the ABA in assists per game in both 1971 and 1972.
Melchionni's #25 jersey was retired by both the Nets and Villanova.
Melchionni's brother, Gary Melchionni, was a two-year captain for Duke University and was an All-ACC performer. Gary's son and Bill's nephew, Lee Melchionni, played for the Duke Blue Devil's from 2002-2006.