Mahoney played three years for Manhattan, accumulating 1,289 points and graduating as the third-leading scorer in school history (currently 16th). He averaged 20.6 points per game as a senior in 1970-1971, which at the time of his graduation was the fourth-best mark in school history, en route to earning All-East Honors. Mahoney also helped lead his fellow Jaspers to victory over North Carolina in the 1970 NIT. Taken in the fifth round of the NBA Draft by the Cleveland Cavaliers, he is one of just 20 student-athletes from the Riverdale-based institution to be drafted by an NBA team and played professionally for the Brooklyn Nets of the ABA during the 1972-1973 season. He also served as the head coach at Manhattan from 1978-1981 before assuming the head coaching reigns at St. John's from 1993-1997, where he earned Big East Coach of the Year honors for the Red Storm. Following his coaching career, he worked in alumni relations for St. John's and currently serves as the color commentator for select Manhattan men's & women's basketball games. He was inducted into the Manhattan College Athletic Hall of Fame in 1998.