Thereafter he had mixed success, winning five fights but losing to Scott Harrison and Mexican world champion Óscar Larios on two occasions. The result of his first fight with Larios is widely disputed. On 17 August 2005 McCullough was appointed the first WBC World Ambassador for Peace and Goodwill in Sports. In September 2005, McCullough became a United States citizen. In November 2005, McCullough released his autobiography, "Pocket Rocket: Don't Quit", in the UK and Ireland. He went on a publicity tour to promote the book, which reached Number 2 on the best sellers list. In 2007, McCullough joined the Ultimate Fighting Championship organisation as a PR associate, to promote Mixed Martial Arts (MMA). He currently trains fighters both in boxing and MMA and is setting up his own charity – IHOW. In 2007 McCullough signed to fight Spain's Kiko Martínez who had just defeated Bernard Dunne at the Point Depot, Dublin for the European super bantamweight title. The fight between McCullough and Martínez was due to take place at Belfast's Kings Hall on 1 December 2007. McCullough had not fought for over two years and the Kings Hall venue was sold out for the fight. It was agreed that the non-title fight would take place at . However, on the day before the fight there was uproar during the weigh-in and the fight was cancelled by the BBBC amid chaotic scenes. McCullough had already contracted to fight at 2 lb over the championship weight and he weighed in at . However, Martínez failed to make the agreed weight and was 1.75 lb over the agreed weight. Martínez was given a couple of hours to shed the excess weight, but did not return to weigh in again and the scales were closed by a BBBC official. A furious McCullough stated "I couldn't believe it. He comes in over the weight and then after being asked to take it off he just sits there and does nothing. I just can't believe what has happened. I was ready to fight and ready to win and he comes in that much over the weight." On 20 June 2008, McCullough fought Juan Ruiz in the Cayman Islands, his first fight in three years. He lost in six rounds, retiring on his stool. Despite being ahead on two of three judges' scorecards after six rounds, he told his corner he could not go on due to an injury he had sustained in training. The Belfast boxer took the microphone and revealed this might be his swansong.