Hamza - tried under the name Mustafa Kamel Mustafa - was found guilty of conspiring to aid terrorist organisations, including al Qaeda. He helped kidnappers of 16 tourists in Yemen in 1998 and tried to build an al Qaeda training camp in Oregon in 1999.
Hamza, who could face life in prison, insisted during testimony that he had never supported terrorism.
But the jury of eight men and four women found the 56-year-old guilty on all 11 counts he faced. US Attorney General Eric Holder described the verdict as "a triumph".
Hamza was extradited in 2012 from the UK, where he led London's Finsbury Park Mosque in the 1990s, reportedly attended by September 11 conspirator Zacarias Moussaoui and shoe bomber Richard Reid. He denied meeting them.
His lawyers argued that he did not participate in any conspiracy and said the US Government's case rested largely on the incendiary language he employed in media interviews and the sermons he gave at the Finsbury Park mosque.