A 6′ 4″ forward[1] who excelled with the hook shot,[2] Hagan, nicknamed "Li’l Abner",[1] played his entire 10-year NBA career (1956–1966) with the St. Louis Hawks.

Hagan played college basketball at the University of Kentucky under legendary coach Adolph Rupp.

After finishing the regular season with a perfect 25–0 record and a #1 ranking in the Associated Press, Kentucky was offered a bid into the NCAA tournament.

After his military service, Hagan and Ed Macauley were traded to the St. Louis Hawks for the draft rights to Bill Russell.

[1] In The Breaks of the Game,[full citation needed] David Halberstam wrote that Hagan was the only white star on the Hawks who welcomed African American teammates like Lenny Wilkens to the team and did not treat them with prejudice.

[1] In 1972, Hagan returned to the University of Kentucky as the school's assistant athletic director and took over the top job in 1975.

He was forced to resign due to recruiting and eligibility violations in November 1988 and was replaced by one-time Kentucky teammate C. M. Newton, the head basketball coach at Vanderbilt University the year before.