Rahman got a relatively late start in the sport. He was an enforcer for drug dealers, and was known for surviving several shootings. He nearly died in a car accident (which left him with permanent scarring on his cheek and ear) and once survived a shooting where five bullets entered his body. 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. He is a devout Muslim.
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 regional USBA heavyweight title, and four months later, he added another regional belt, the IBF Intercontinental heavyweight title. Defending the USBA title three times and the Intercontinental belt twice. 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 the IBF's mandatory contender. Rahman was using his power jab well, out boxing Tua virtually every round. At the end of the 9th round Tua staggered Rahman with a devastating punch that dazed him. At the beginning of the next round Tua pounced on him immediately and the referee jumped in when Rahman was bobbing and weaving. Tua won by TKO. It was argued that it should have been a DQ.
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. At one point during the match, Rahman was knocked through the ropes onto the floor, hitting his head on the floor. Rahman dropped out of The Ring top 10 as a result of the surprise loss. Rahman later said that he had seen Maskaev earlier in his career get knocked out in the first round by former champion Oliver McCall and he assumed he was brought in as an easy win. Because of this, he did not train as hard as he should have and was beaten.
Rahman came back with three wins, including one in May 2000 over Corrie Sanders in a war in which he was dropped twice and came back to win in 7, and after that he was moved back up in the rankings. After this fight, Sanders briefly retired. Finally, on April 22, 2001, Rahman earned a shot at Lennox Lewis the WBC, IBO and IBF heavyweight champion with a win over Frankie Swindell. Rahman beat Swindell via 7th-round RTD. Swindell quit on his stool after the 7th round, informing the ringside doctor that he had suffered an eye injury.
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 defeating Lewis via 5th-round KO. It was only the second loss of Lewis' career. His first was on September 24, 1994, when Oliver McCall defeated him via 2nd-round TKO.
Lewis had an immediate-rematch clause in the contract for his defense against Hasim, and chose to invoke it. Rahman and his new promoter, Don King, made plans to defend the titles against David Izon, rather than giving Lewis his rematch. 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, the two men met again. This time, Lewis knocked Rahman out in the fourth round. For the rematch, Lewis made $11 million and Rahman got $10 million. The fight generated 460,000 pay-per-view buys and $23 million in revenue.
Rahman's comeback fight ended in disappointment, he was beaten by a technical decision by aging former champion Evander Holyfield after headbutts from Holyfield caused a massive swelling (severe hematoma) on Rahman's forehead.
On March 29, 2003, Rahman faced David 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. Tua was knocked down a split second after the bell rang and it was not ruled an official knockdown. Rahman came in at the heaviest weight of his career. 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 WBA heavyweight title (caused by Roy Jones, Jr. deciding whether he wanted to stay a heavyweight or return to Light Heavyweight). Rahman was favored, but he lost by (twelve round) unanimous decision and again was out of condition. Ruiz, who became interim titleholder with this victory, was upgraded to WBA heavyweight champion after Roy Jones Jr. vacated the title on February 20, 2004, choosing to fight Antonio Tarver for the WBC and IBO light heavyweight titles (already in 2003), rather than Ruiz.
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, with the winner being 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 US$4.2 million.
After this third rescheduling, Rahman, fought for a WBC "Interim" heavyweight championship and defeated Monte Barrett on August 13, 2005, via unanimous decision.
At this point, Klitschko would be stripped of his 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, making Rahman a two-time heavyweight champion.
On December 9, 2005, a U.S. Bankruptcy Court judge ended Rahman's contract with Don King and he signed instead with Top Rank Boxing.
On March 18, 2006, Rahman fought James Toney to a draw, in a 12-round fight in Atlantic City, New Jersey, and he retained the WBC heavyweight title. The judges scorecards read 114-114, 117-111 and 114-114. Judges Tom Kaczmarek and Nobuaki Uratani had Toney leading by one point heading into the 12th. Rahman only retained the title by winning the final round.
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. Rahman was knocked down early in the 12th round and eventually stumbled to the mat. Rahman was then holding on to the ropes to stay on his feet. Maskaev landed a flurry of punches with Rahman unable to defend himself. The referee stopped the bout to save Rahman from further punishment. Afterwards he said he should have followed his trainer's game plan.
After a 10-month layoff, Rahman resumed his career with a ten-round unanimous decision over Taurus Sykes June 14, 2007, at The Main Street Armory in Rochester, New York. Rahman then fought three times in three months defeating Dicky Ryan by second-round TKO on September 7, 2007. Hasim followed up by stopping Cerrone Fox on October 18, 2007. On November 15, 2007, Rahman then scored a 10th-round TKO for the NABF title against Zuri Lawrence. In this fight, Lawrence fell through the ropes on two occasions whilst ducking punches, once in round six and again in round nine. In round six, he landed on the floor below the ring, but beat the referee's 20-count. He received a standing ovtion from the crown when he re-entered the ring.
On July 16, 2008, Rahman was stopped by an accidental headbutt against James Toney. Toney was originally awarded a TKO victory as the referee declared Rahman had quit between rounds, a week later, the TKO victory was overturned and changed to a No Contest due to the headbutt. After three rounds, Toney was ahead on 29-28 on two of the judges cards while Rahman was ahead, 29-28 on the third card.
Rahman lost to Wladimir Klitschko (TKO in the 7th round) on December 13, 2008, for his WBO, IBO & IBF titles in a fight some believe showed himself to be past his best. Klitschko dominated the fight, winning every round while making good use of his left jab. Rahman seemed unable to withstand the Ukrainian's punch power. The referee called a stop to the contest in the 7th round after Rahman failed to respond to a series of shots.
In an interview dated February 26, 2009, with DreamFighters.com, Rahman expressed his interest in crossing over into mixed martial arts.
Fifteen months after the seventh round loss to Wladimir Klitschko in December 2008, Rahman made a comeback against 43-year-old journeyman Clinton Boldridge winning the fight via TKO 1. Rahman next fought on June 19, 2010, against journeyman Shannon Miller (16–4, 9 KO) in a match that took in the Niagara Falls Conference Centre, Niagara Falls, New York, United States. Rahman came into the ring at 260 pounds and won the bout by TKO in the 4th round, after sending Miller down in each of the four rounds. He then won his next three fights (all by knockout), against Damon Reed, Marcus McGee and Galen Brown. His win over Galen Brown marked Rahmans 50th professional win in his career. He is now 5-0 (all wins via knock out) since his loss to Wladimir Klitschko.
He was elevated to the WBA number one contender to face Alexander Povetkin on September 29, 2012, for the WBA regular heavyweight championship. Povetkin beat Rahman via 2nd-round TKO.
In yet another attempt to re-enter the heavyweight scene Rahman signed up for the super 8 tournament, an elimination-style event staged in Auckland, New Zealand. But despite holding the tag as tournament favorite the ageing Rahman was outpointed by little-known club fighter Anthony Nansen in the quarter-final on the 4th June 2014.