A Saudi man described by prosecutors as one of Osama bin Laden's most trusted lieutenants was convicted in a federal court in New York in connection with the 1998 bombings of US embassies in Kenya and Tanzania.
Khalid al-Fawwaz, 52, faces up to life in prison after a jury on Thursday convicted him on four conspiracy counts on their third day of deliberations.
The bombings killed 224 people and injured more than 4,000.
"We hope this verdict gives some comfort to al-Qaeda's victims around the world," US Attorney Preet Bharara said in a statement.
Fawwaz was not charged with planning the attacks.
Instead, prosecutors said he functioned as a key bin Laden associate while living in London, disseminating the al-Qaeda leader's declarations of war to the media and sending equipment to al-Qaeda members in Africa.
Fawwaz was also accused of operating an al-Qaeda training camp in Afghanistan in the early 1990s and helping lead an al-Qaeda cell in Kenya's capital, Nairobi, that later conducted surveillance ahead of the embassy bombing there.
A defence attorney, Bobbi Sternheim, said Fawwaz would appeal.
"Trying a pre-9/11 case in a post-9/11 era within blocks of the World Trade Center insured Mr Fawwaz would never receive a fair trial from an impartial jury," she said, referring to the September 11, 2001, attacks in downtown Manhattan.
Fawwaz's attorneys painted their client as a peaceful dissident who shared with bin Laden a desire to effect reform in their native Saudi Arabia, but turned away from him when he began calling for violence against US civilians.
Prosecutors, however, said Fawwaz did whatever was asked of him to help advance al-Qaeda's mission.
Fawwaz was arrested in London in 1998 and extradited to the US in 2012 following a lengthy legal battle.