They agreed to sell "huge quantities of serious weapons to what they believed was a terrorist organisation who would use these weapons, amongst other things, to kill Americans and wreak havoc", the judge said.
Moreno Godoy, a 60-year-old Chilean, was sentenced to 25 years in prison.
A federal jury convicted al-Kassar of masterminding the deal that included 15 surface-to-air missiles, and thousands of assault and sniper rifles and rocket propelled grenade launchers for a $1m profit.
Prosecutors said he made the deal knowing the Farc would use the weapons against US helicopters and citizens to dissuade American efforts and disrupt the cocaine trade.
During the sentencing, al-Kassar asked for leniency by quoting Jesus Christ, the Koran and an old Arabic poem before saying he was not "against Americans, America or against any other kind of nations".
The prosecution case was based largely on evidence gathered by two undercover operatives who posed as Farc arms buyers and videotaped negotiations in Spain with al-Kassar and Moreno.