According to media reports, al-Fadhli was a close confidant of Osama bin Laden, and one of a few people to be informed of the September 11 attacks, before they were launched. He was born in Kuwait and used the aliases Abu Majid Samiyah, Abu Samia, Dawud al-Asadi, Muhsin Fadhil Ayyid al-Fadhli and Muhsin Fadil Ayid Ashur al-Fadhli. In 2012, the United States State Department identified al-Fadhli as the leader of al-Qaeda in Iran and issued a $7 million reward for his capture.
In mid-2013, al-Fadhli was sent to Syria on behalf of al-Qaeda emir, Ayman al-Zawahiri, in order to mediate disputes between al-Qaeda's then Iraqi branch, the Islamic State of Iraq and the Levant (ISIL) and the al-Nusra Front. In February 2014, al-Fadhli was instrumental in influencing al-Qaeda to disassociate itself from ISIL.
Al-Fadhli was killed on 8 July 2015, when the United States carried out an airstrike, targeting a vehicle near the town of Sarmada in northwest Syria, according to the Pentagon.