The men face up to life in prison when they are sentenced on February 18.
Lawyers for both men said they would appeal.
No death sentence
Al-Kassar, 63, also dubbed the "Prince of Marbella" for his lavish lifestyle in Spain, was extradited from Spain to the US after Washington agreed not to impose the death sentence if he were found guilty.
"[This] puts an end to his ability to command a global munitions empire that armed and funded terrorist organisations for decades"
Michele Leonhart, acting administrator for US Drug Enforcement Agency
The two men were caught in a sting operation by US authorities as they plotted to sell missiles, machine guns and ammunition to Colombia's Farc, which the US lists as a terrorist group.
Prosecutors said al-Kassar and his associate organised a deal worth millions of dollars in multiple discussions and offered to provide tonnes of explosives, guerrilla warfare experts from Lebanon and other military aid to Farc.
US officials said al-Kassar had been dealing in arms since the 1970s and had clients from Central America to the former Yugoslavia to Iraq, including the Palestinian Liberation Front armed group.
Al-Kassar was acquitted in 1995 after a three-year trial of supplying weapons used in the hijacking of the cruise ship Achille Lauro, during which a wheelchair-bound US passenger, Leon Klinghoffer, was murdered.
The Anti-Defamation League (ADL), a Jewish lobby, welcomed al-Kassar's conviction, saying that "justice has finally caught up" with him.