The 12-person jury returned the verdict after two days of deliberations following a five-week trial in New York.
Manhattan attorney Preet Bharara welcomed the verdict, saying that for nearly a decade, Fawwaz had played "a critical role for Al-Qaeda in its murderous conspiracy against America.
"Fawwaz was one of Osama bin Laden's original and most trusted lieutenants, serving first as the leader of an Al-Qaeda training camp in Afghanistan, then as a leader of Al-Qaeda's terrorist cell in Kenya, and finally as bin Laden's media adviser in London," he said.