Despite declaring after the 2004 Olympics that he would pursue a Gold medal at the 2008 Beijing Olympics, Khan turned professional in 2005, signing with England's leading boxing promoter, Frank Warren. This was fuelled by a row with the English Amateur Boxing Association over the ticketing allocation for his family and friends at the English ABA Finals.
Khan moved from Lightweight to Light Welterweight for a single fight against French fighter Rachid Drilzane on December 9, 2006, winning a 10 round decision. Khan experienced the first knockdown of his career; he got his foot caught in the rope and touched down with his glove, giving the referee no choice but to award his opponent the knock-down. Drilzane had never scored a knockout in his 13 fight career. Khan did not defend this belt — instead he returned to the Lightweight division for future fights.
Khan announced his intention to be a World Champion by the time he was 21, by the end of 2007 — an ambition that was not to be fulfilled. His comments prompted reactions from boxing fans directed towards his promoter to increase the level of his opposition.
On April 7, 2007, he beat Steffy Bull in a three round technical knockout in Cardiff, which was later criticized by many observers to have been fixed, but the remarks were later polished by defending promoter Frank Warren.
On July 14, 2007 Khan faced Willie Limond for the Commonwealth lightweight title. During the fight Khan was knocked down in the 6th round and appeared to be hurt badly. However, after another barrage of right hands, Khan weathered the storm and in the seventh round knocked down Limond after a fierce exchange of punches. The referee stopped the fight on advice of the doctor; Limond had sustained a perforated eardrum, broken nose and damaged right eye.
On October 6, 2007 Khan faced Scott Lawton in his first defence of the Commonwealth Lightweight Title. Khan fought a mature and clinical fight, boxing with single punches for the majority of the first three rounds. Khan increased the pressure at the end of the third, and secured a TKO victory in the fourth. The referee stepped in when Lawton failed to fight back.
The most significant win of Amir's career so far came when Khan celebrated his 21st birthday by successfully defending his Commonwealth Lightweight Title against Graham Earl on December 8, 2007. Earl, rated the number one British lightweight and a former world title challenger, was considered Khan's toughest test by some way and a tough fight was expected — especially due to some ill-feeling between the pair in the run-up to the fight. However, it took Khan just 72 seconds to have the fight referee declare Earl in no fit state to continue. The speed with which Amir dispatched his man came as a big surprise to most observers, Earl looking barely in the same ring as the Commonwealth champion. Khan later claimed that he rated this victory as the best of his career.
Khan beat Australian Gairy St Clair on February 2, 2008 in a contest for the Commonwealth lightweight title at the ExCel Arena in London. This was his first fight to last all 12 rounds and was won via a unanimous 120-108 scoring from all three ringside judges.
On April 5th 2008, Khan is set to fight Martin Kristjansen. The winner will determine the contender for the WBO World lightweight title.
Amir Khan becomes World Champion 2009!