Mullins was a member of the United States Olympic basketball team that won the gold at the 1964 Summer Olympics.
Mullins was taken by the St. Louis Hawks in the first round (6th pick overall) of the 1964 NBA Draft. After two lackluster seasons with the Hawks he moved to the Golden State Warriors where he enjoyed the best seasons of his career and was selected as an NBA All-Star three times - in 1969, 1970, and 1971. He helped the Warriors to the 1967 Western Conference title and the 1975 NBA championship. Upon his retirement in 1976 he had amassed a total of 13,017 points for a twelve-year career average of 16.2 points per game.
In 1985, Mullins was hired as the head men's basketball coach and athletic director at UNC Charlotte. The program had struggled since making the NCAA Final Four in 1977, and in three years Mullins took the 49ers back to the NCAA Tournament for the first time since their 1977 run. His 182 victories over eleven seasons stood as a school record until Bobby Lutz, Mullins' former assistant coach, surpassed that total in 2008.