Walid bin Attash was born in 1979 to a family of Yemenis that were forced to flee to Saudi Arabia due to his father's radical Islamist views. Many of Walid's brothers fought in the Afghan Civil War in the 1990s, and Walid attended the University of Islamic Studies in Karachi, Pakistan, not far away. Walid later fought in the war, and in 1997 he lost a leg fighting the Northern Alliance as a member of al-Qaeda; his brother died in the same battle. His first role as an al-Qaeda member was the planning of the 1998 US embassy attacks in Dar-es-Salaam, Tanzania and Mombasa, Kenya, serving as the cell chief in Nairobi and passing messages between the Sheikh Bin Laden and Mohammed Atef, his third-in-command. In 1999 he was tasked with obtaining explosives for the attack on USS The Sullivans as a part of the Millennium Plot. In October 2000 he was identified as the mastermind behind the bombing of the USS Cole off the coast of Yemen, and he served as Osama bin Laden's bodyguard soon after, and in this position he personally selected some of the 9/11 hijackers.
On 29 April 2003 Walid and Ammar al-Baluchi were both captured in Karachi and sent to "The Dark Prison", where he was tortured. On 6 September 2006 Walid and 13 other high-ranking al-Qaeda leaders were transferred to the Guantanamo Bay Internment Camp, and in 2010 a trial was set up; on 30 October 2015 his request to drop his legal counsel was denied and the trial moved forwards.