He was to fight WBA champion Aaron Pryor in the summer of 1982 for a unification bout in the 140-lb. division, but instead fought and lost (both match and title) by split-decision to Leroy Haley in June of that year. He would play the role of world title challenger twice more, once in a rematch with Haley in February 1983 (losing a 12-round un. dec.) and once in a challenge to new champion Billy Costello in November 1984 (dropping another 12-round un. dec.) In fact, Mamby replaced Haley with five days to go before the Costello bout - Haley backing out due to a claimed hand injury. While Costello's camp was afraid of Mamby's ring savvy and crafty veteran status, the fight went ahead anyway. That would be Mamby's last title bout. Other names Mamby has fought include Roberto Duran (losing by points in a non-title affair in 1976) and Saengsak Muangsurin (losing a 15-round decision in Thailand in 1977 in an attempt to win Muangsurin's title, the same WBC 140-lb. belt Mamby would win from Saengsak's conqueror, Sang-Hyun).