As the inaugural coach of the Bucks, he led them to a championship in their third season of existence in 1971, the fastest run for an expansion team in NBA history.

He led the Purple Eagles to the National Invitational Tournament in 1953 and 1954 before graduating as the all-time leading scorer (1,275) in program history (he now ranks in the top thirty).

Against Siena on February 21, 1953, Costello played all but twenty seconds of a six-overtime game that ran for 69 minutes, where he scored 21 points in an 88–81 win.

[5][6] Costello was drafted by the Philadelphia Warriors in 1954, electing to choose it over study at the University of Buffalo and their dental school due to having more of a passion for basketball.

Costello retired in 1965 from the Philadelphia 76ers (the former Syracuse Nationals), but eventually came back for the 1966–67 NBA season after new head coach Alex Hannum told him he needed a veteran point guard.

Forty-two games into the season, Costello tore his Achilles tendon on January 6, 1967, and was replaced by Wali Jones.

Costello began his coaching career at East Syracuse-Minoa High School, his alma mater, where he served for the 1968 season.

In his second year in Division I, the Pioneers were the seventh most improved team in the country based on their won-loss record.