The drugs were seized by coastguards from a cargo vessel after a gun- battle off the coast of Washington state.
Forwell, originally from Burnham, Buckinghamshire, was one of the last defendants in the case to come to trial. A total of 17 people have now been convicted in relation to the investigation.
He had gone underground for four and a half years after the seizure, before being arrested in London in 1993. His elusiveness and creation of numerous false identities earned him the nickname The Fox.
His lavish jet-set lifestyle was finally brought to an end when officers arrested him as he wandered around a north London antiques market.
Investigations have established that by 1979 he had become a major international trafficker in marijuana.
Operating out of Singapore, Forwell oversaw the shipment of massive quantities of marijuana from south-east Asia on board cargo vessels to the US west coast.
Passing sentence, judge Susan Barnes said: "This sentence should be a strong message to anyone considering becoming involved in narcotic trafficking in Washington that there is a heavy price to pay for such criminal conduct, and that there is no place in the world to hide." She commended the work of special agents and police, including Det Sgt David Jones of the Metropolitan force.Reuse content