Rahman grew up in Baltimore, but unlike most championship boxers, got a relatively late start in the sport. He was an enforcer for drug dealers, and in his section of the city he was portayed as a young Frank Lucas, he's known for surviving several shootings. 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 determine who the IBF's mandatory contender would be. Rahman was fighting well 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 only after the normal one minute break in between rounds. Tua pounced on him immediately, and the ref jumped in when Rahman was bobbing and weaving, but not punching back and sitting on the ropes.
Because of the controversial nature of the loss, Rahman's ranking did not suffer, but in November 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.
In the fight, held at Brakpan, South Africa, Lewis and Rahman traded hard blows for five rounds before Rahman, a 20-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 the third Muslim, after Muhammad Ali and Mike Tyson, to hold a world heavyweight championship. (Since 2001, the list has grown to include Ruslan Chagaev and Sultan Ibragimov.)
Lewis had an immediate-rematch clause in the contract for his defense against Hasim, and chose to invoke it. Rahman and his new promoter, Omar Murtaza, made plans to defend the titles against at least one other opponent first, and give Lewis his rematch later. Lewis sued Rahman in U.S. federal court to enforce their contract. The judge, Neil McCluskey sided with Lewis and on November 17, 2001,in Las Vegas, Nevada, the two men met again. This time, Lewis knocked Rahman out in the fourth round.
On March 29, 2003, Rahman faced Tua for a second time. The fight was ruled a draw after one judge scored it for Rahman, a second for Tua and a third had the score even. In June, Rahman was elevated to the No. 1 contender's position by the WBC. On December 13, Rahman was matched with former WBA world champion John Ruiz, in a match for an Interim (i.e., temporary, pending the inactive champion's return) WBA heavyweight title. Rahman was favored, but he lost by (twelve round) unanimous decision.
After this defeat, Rahman stepped back to a lower level of competition and defeated four journeyman fighters while working to get back into shape. His efforts paid off when he was rewarded with a fight against Kali Meehan on November 13, 2004. The fight was an elimination bout for the IBF, WBA, and WBC, so the victor could potentially become the number-one contender in more than one world-title organization. Rahman scored the victory by a fourth-round knockout at New York City's Madison Square Garden.
The WBC designated Rahman as Vitali Klitschko’s next mandatory challenger; the fight was set for April 30, 2005. Klitschko injured his thigh while training for the fight, so it was rescheduled for June 18. As this date approached, Klitschko’s camp said that the thigh had not fully healed; the WBC made July 23 the new fight date. Soon after this second postponement, Vitali’s doctors reportedly discovered back injuries that they said demanded minor-yet-immediate corrective surgery. The WBC pushed Rahman's title shot back again, this time to November 12; Rahman's share of the purse following this match would reportedly be around $4.2 million (U.S.).
At this point, Klitschko would be stripped of his now-disputed WBC title if his first fight back was not against Rahman. On November 7, it was announced that Klitschko had suffered severe right knee injuries during training; the WBC said it would strip him of the championship if he was unable to box within 60-90 days of a soon-to-be-announced base date. However, on November 9, Vitali Klitschko retired instead. On November 10, 2005, the WBC voted to award its heavyweight championship to Rahman.
On March 18, 2006, Rahman fought James Toney to a draw, in 12-round fight in Atlantic City, New Jersey, but he retained the WBC heavyweight title. On August 12, 2006, Rahman lost the WBC heavyweight championship to rival Oleg Maskaev by 12th Round TKO in a mandatory defense of his title.
After a long layoff, Rahman resumed his career with a ten-round unanimous decision over clubfighter Taurus Sykes June 14, 2007, at The Main Street Armory in Rochester, New York. He had won four more fights until being stopped by James Toney.
On July 16, 2008 James Toney was originally awarded a TKO victory over Hasim Rahman in the third round after the ringside doctor stopped the fight due to an accidental head butt. Rahman was cut after colliding heads with Toney, and referee Ray Corona ended the rematch. However, appeals were filed by Rahman's camp as fight rules stated that in case of a fight stoppage due to a head butt before the end of the 4th round, the fight was to be called a no-contest. After a thorough review of the bout, on July 28, 2008, the outcome was reversed and the fight was declared a no-contest by the California State Athletic Commission,due to the accidental headbutt which caused a stoppage of the contest.