Rahman grew up in Baltimore, but unlike most championship boxers, got a relatively late start in the sport. He took up boxing at age 20 and had just 10 amateur bouts before making his pro debut on December 3, 1994 at age 22.
Despite his inexperience, Rahman had obvious natural boxing skills that propelled him to 11 knockout wins in his first 12 fights. Then he took a step up in class in March 1996 with a 10-round decision win over veteran Ross Puritty and seven months later, he repeated the feat against former world champion Trevor Berbick.
In July 1997, he won the USBA regional heavyweight title, and four months later, he added another regional belt, the IBF Intercontinental heavyweight title. By the fall of 1998, he was ranked as one of the top five heavyweights in the world.
On December 19, 1998, Rahman faced fellow contender David Tua in a fight to determnine who the IBF's mandatory contender would be. Rahman apparently won most of the first eight rounds and appeared to be on his way to victory when Tua staggered him with a punch after the bell in the ninth round. Because the punch was illegal, Rahman should have been given whatever time he needed to recover, but the referee erroneously forced him to begin the 10th round after just a few minutes. Tua pounced on on him immediately and won by a technical knockout.
Because of the controversial nature of the loss, Rahman's ranking did not suffer, but in November of 1999, he was knocked out by Oleg Maskaev in the eighth round of a fight he looked to be winning, and dropped out of the Ring Magazine top 10 as a result.
Rahman came back with three wins, including one in May of 2000 over Corrie Sanders for the fringe WBU heavyweight title, and moved back up in the rankings. Finally, on April 22, 2001, Rahman earned a shot at Lennox Lewis' world heavyweight title.
In the fight, held at Brakpan, South Africa, Lewis and Rahman traded hard blows for five rounds before Rahman, a 14-to-1 underdog, stunned the crowd by knocking Lewis out with one punch. It was only the second loss of Lewis' career, and made Rahman only the second Muslim ever (after Muhammad Ali) to hold the world Heavyweight championship.
Lewis had a clause in the fight that he could get an immediate rematch if he lost, and chose to invoke it. So on November 17 in Las Vegas, the two men met again. This time, Lewis landed the devastating punch in another slugging match, sending Rahman down for the count in the fourth round.
Rahman's next fight took place on June 1, 2002 against former champion Evander Holyfield, as both men sought another shot at Lewis, or failing that, a chance to fight for the WBA belt that Lewis had vacated. An accidental headbutt by Holyfield on Rahman ended the fight after just seven rounds, and sent it to the scorecards. Rahman lost a split decision.
On March 29, 2003, Rahman faced Tua for a second time, and again the fight ended in controversy. The fight was ruled a draw after one judge scored it for Rahman, a second for Tua and a third had the score even. However, most observers thought Rahman won the fight, and in June, he was elevated to the No. 1 contender's position by the WBC, putting him in line for an eventual rubber match with Lewis. However, he then suffered another setback, losing to former world champion John Ruiz by a twelve round unanimous decision on December 13.
His career record stand at 35 wins, five losses and one draw, with 29 knockouts.