McClellan turned professional in 1988. Trained by hall of fame trainer Emanuel Steward, he captured the vacant WBO middleweight title by knocking out John Mugabi in one round in 1991, and the WBC middleweight title by knocking out Julian Jackson in five rounds in May 1993. He defended the WBC title three times, all first round stoppages, including a rematch with Jackson.
McClellan moved up in weight to challenge WBC super middleweight champion Nigel Benn in London on February 25, 1995. The fight was watched by an estimated 17 million people on television and 10,300 paying spectators.
In a savage bout, McClellan knocked Benn out of the ring in round one and scored another knockdown in round eight, but each time Benn was able to work his way back into the fight and kept landing hard power punches to the challenger. Referee Alfred Azaro was also roundly criticized for his officiating mistakes, which included impeding the challenger's progress when McClellan was trying to finish off the champion. McClellan was noticeably blinking repeatedly early in round ten, during which, after receiving a single hard blow from Benn, he voluntarily went down, taking a knee. McClellan took the mandatory eight count and the fight was resumed, but he did not throw another punch and moments later he dropped to his knee for a second time and allowed Alzaro to count him out. The fight over, McClellan immediately stood and walked to his corner under his own power. He sat on the canvas leaning against the ring apron, but while being attended to by ring physicians he slumped onto his back and lost consciousness. McClellan was strapped to a stretcher and rushed to the hospital.
McClellan had emergency surgery to remove a blood clot from his brain. He spent eleven days in a coma and he was found to have suffered extensive brain damage. He lost his eyesight, the ability to walk unassisted and was reported as being 80 percent deaf. Sports Illustrated ran an article about the fight and its outcome one week after the fight. McClellan's family flew to be by his side, and later he was flown back to his home country. He has recently recovered some ability to walk, being helped by a cane, but he has not recovered his eyesight. In addition to being blind, his short-term memory was also profoundly affected. His three sisters, particularly Lisa McClellan, are responsible for his care. In a 2011 documentary broadcast by ITV (who originally screened the fight live in the UK), Lisa stated that Gerald is in fact not deaf, but that he has trouble with comprehension when spoken to.
Tarick Salmaci, a Kronk Gym fighter claimed later in an interview that he sparred with McClellan some time before the Benn fight, and after being hit with a jab, McClellan started to blink hard and the session had to be stopped. McClellan initially claimed that he was thumbed but later admitted in the locker room that he was in fact seriously hurt. Salmaci said that he it found strange that a fighter with McClellan's chin, wearing a headgear is being hurt by a jab, and that when he noticed McClellan blinking during the Benn fight the same way, he was immediately aware that he was in serious trouble.
McClellan has been the honoree at numerous banquets and award ceremonies, and fellow boxing world champion Roy Jones Jr., often pointed out as a rival middleweight champion during 1993–94 (indeed, McClellan actually beat Jones as an amateur), set up a foundation to help McClellan.
Nigel Benn himself has also helped to raise funds for McClellan's treatment, and the two men would meet again for the first time since their bout at a fundraiser held in London on February 24, 2007. Several items were auctioned off at the event and a total of £200,000 was raised. In December 2011, McClellan's family announced they were putting McClellan's world title belts up for auction to pay for his continuing round-the-clock care, adapt his house for his changing medical needs and avoid having to place him in a nursing home. Told by his sister Lisa about what she needed to do to maintain his quality of life and keep him at home, McClellan, who remains 80% deaf, was surprisingly able to hear and understand his sister and gave his approval. Gerald told his sister, "Sister, just do what you've gotta do."
In May 2012, the World Boxing Council publicly appealed for donations to a trust fund set up in McClellan's name in order to help his sisters maintain his 24-hour care. In July 2012, McClellan took a turn for the worse, and underwent surgery to remove his colon. Former world light middleweight champion Terry Norris, whose Final Fight Foundation acts to protect boxers, made an appeal for the Gerald McClellan Trust, noting, "McClellan's organs are starting to shut down because of his brain injury." Ring 10, a nonprofit organization that helps impoverished former fighters, provides McClellan with a monthly food credit and raises funds to assist in payment of other necessities.
According to an article in The Observer, McClellan participated in dog fighting. McClellan's trainer and family admitted that McClellan was involved with fighting pitbulls, and on one occasion had used tape to bind the jaws of a Labrador shut before allowing his pet pitbull "Deuce" to kill it.