Moorer is a native of Boca Raton, Florida, but for most of his career, he trained under the tutelage of hall of fame trainer Emanuel Steward in Detroit. As a member of Steward's Kronk Gym team, he was teamed up with such boxers as Thomas Hearns and Gerald McClellan.
Moorer had a fast but steady rise through the professional boxing ranks. He debuted on March 4, 1988, knocking out Adrian Riggs in the first round. Before the year ended, he was undefeated in eleven bouts and fighting for the world title for the first time. He conquered the vacant WBO world Light-Heavyweight title with a five round knockout of Ramsi Hassan.
In 1990, his relationship with Steward began to suffer, and eventually, Moorer went over to Teddy Atlas. He retained the title three times before the end of the year, beating Mario Melo and former Michael Spinks challenger Jim McDonald, among others.
He won all his fights of 1991, year in which he decided to campaign at the Heavyweight division. Then, in 1992, he fought for the vacant WBO world Heavyweight championship. He knocked out former Evander Holyfield challenger Bert Cooper in the fifth round, making him only the second man to go directly from holding a world light heavyweight title to holding a heavyweight title. That night, he also became the first left-handed boxer in history to become a world heavyweight champion.
He did not defend the WBO heavyweight belt. Instead, he spent 1993 making a few preparation bouts, training to challenge Holyfield, who was considered the true world heavyweight champion, for his IBF and WBA belts. On April 22, 1994, before a Pay Per View audience, Moorer overcame a second round knockdown and went on to win a majority decision over Holyfield.
In his first defense of those belts, November 10 of the same year, Moorer was ahead on all three judges' scorecards, when he received a right hand to the chin by George Foreman in round 10, getting knocked out and losing the world championship. In addition to the belts, he also lost his undefeated record that night.
Moorer re-grouped by winning one fight in 1995. Foreman retained the title with a close and controversial decision against Axel Schultz. Because of the controversial nature of the Foreman-Schultz bout, the IBF ordered Foreman to travel to Germany for a rematch, but Foreman refused, choosing to leave the IBF belt vacant instead. South African Frans Botha travelled to Germany instead and beat Schultz with another close decision to claim the title, but he was stripped of the title when he was discovered with drugs shortly after. Moorer then went to Berlin to fight Schultz for the vacant crown, and on June 22, 1996, Moorer won the IBF heavyweight crown once again, by beating Schultz by a 12 round decision.
His first defense, against former world champion Botha, came on November 9, 1996. It was, according to Ring Magazine, one of the fights of the year. Moorer recovered from a knockdown against him to lead on two of the three scorcards going into the 12th and final round, then ended with a flourish, knocking Botha out with just 18 seconds left in the bout.
He had two fights in 1997, retaining his belt with a 12 round decision over Vaughn Bean, and then losing to Holyfield by a knockout in eight rounds in November. Moorer visited the canvas five times in his rematch with Holyfield.
After this, he retired from boxing for three years before returning with a knockout of journeyman Lorenzo Boyd. He won three more fights, then seemingly retired again when he was knocked out in only 30 seconds of round one by David Tua on August 17, 2002. However, he returned to the ring once again on March 29, 2003, beating Otis Tisdale on points over 10 rounds, and at age 35, is seeking a shot at his fifth world championship belt. On August 23, he beat Brazil's Rodolfo Lobo by knockout in only 64 seconds.
Moorer and Atlas didn't always enjoy the best of relationships either, Atlas claiming that Moorer was hard to inspire when he was fighting. There are videos that show Atlas telling Moorer that Moorer's little son had just called and was crying on the phone, while Moorer was in the middle of his first bout with Holyfield. After his loss to Holyfield, Moorer fired Atlas and replaced him with Freddie Roach.
Moorer scored knockouts in each of his first 29 bouts, placing him in the exclusive list of boxers who have won at least 20 fights in a row by knockout, alongside such other fighters as Foreman, Wilfredo Gomez, Carlos Zarate, John Mugabi, Khaosai Galaxy and Aaron Pryor.
As of August 2003, his record stands at 45 wins, 3 losses and 1 draw, with 35 wins by knockout.