The Jordanian-born fighter rose to prominence as leader of the Islamist Tawhid and Jihad group in 2003.
In 2004, al-Zarqawi announced that he pledged allegiance to Osama bin Laden and changed the organisation's name to al-Qaeda in Iraq.
The group carried out some of the most deadly attacks in Iraq since the US-led invasion, including the January 2005 bombing of a crowd of police and Iraqi National Guard recruits in the southern city of Hilla that killed 125 people.
Al-Zarqawi is alleged to have personally beheaded at least two American hostages during 2004 - Nick Berg and Eugene Armstrong.
During 2005, al-Qaeda in Iraq began to move their campaign beyond Iraq's borders - carrying out a suicide attack on a Jordanian hotel that killed 60 people and claiming responsibility for a rocket attack against Israel.
The US put a $25 million bounty on his head, the same as for Osama bin Laden, and al-Zarqawi was sentenced to death three times in his native Jordan.
His group was also at the centre of the Iraqi sectarian conflict that has threatened to develop into all-out civil war.
Al-Qaeda in Iraq claimed responsibility for the bombing of Shia mosques and al-Zarqawi described Shia Muslims as "enemies of Islam" in an audiotape posted on the internet in June.
But analysts believe that despite being a prominent figure in the Iraqi uprising, his influence was often exaggerated by the media.
Al-Zarqawi adopted his radical
Islamist ideology while in prison
His organisation was believed to be only 3,000 strong at most and US army officials admitted raising al-Zarqawi's profile by blaming attacks on his group, the Washington Post reported in April.
After reports that he had been dislodged as political leader of the Iraqi uprising, al-Zarqawi released a video in May in an attempt to maintain his profile - a move that may have provided the US with information on his whereabouts.
Born Ahmad Fadhil Nazzal al-Khalayla in 1966, al-Zarqawi was known in the Jordanian industrial town of al-Zarqa as a small-time criminal.
He adopted his Islamist radical ideology while in a Jordanian prison in the late 1990s.
After being released in an amnesty, al-Zarqawi went in 1999 to Afghanistan, where he formed links with bin Laden.
He fled during the US-led war that toppled the Taliban government in late 2001, passing through Iran to Iraq, according to US officials.