Three judges delivered the verdicts after a nine-month trial involving 19 accused members of November 17.
There was no jury because terrorism charges were involved.
Four of the 19 defendants were found not guilty including the only woman, the wife of the convicted chief assassin.
The convictions and cracking of the group's network removed a major security concern in advance of next summer's Olympic Games in Athens.
The mastermind, Alexandros Giotopoulos, 59, was found guilty of plotting several murders, including the last killing, a drive-by shooting of British military attache, Stephen Saunders in 2000.
Saunders' widow, Heather, said: "At the end of the day nobody really wins in this situation, but if they are taken off the streets for a while and given a dose of their own medicine - albeit no comparison to what we suffered - then that, perhaps, is justice."
Giotopoulos is a mathematician who was a student in Paris in the 1960's and is the son of Greece's most prominent Trotskyite.
As he was led from the court after being convicted of nearly
1000 charges, he smiled, waved to spectators and shouted: "Today's Greece is a modern colony of the United States."
The court found beekeeper Dimitris Koufodinas, the main hitman, guilty of Saunders' murder.
It was one of 253 charges against the cold-blooded killer, known as "Poison Hand" for his close-up murders with a pistol.
"We are not interested in this decision. This verdict does not concern us, we are only interested in the judgement of history and of the Greek people."
Koufodinas shouted: "We are not interested in this decision.
This verdict does not concern us, we are only interested in the judgement of history and of the Greek people."
November 17, a radical Marxist group, was named after the date of a 1973 student uprising which was crushed with tanks by Greece's then ruling military.
The group claimed responsibility for murdering 23 Greeks as well as British, US and Turkish diplomats that began with the 1975 killing of Athens CIA station chief, Richard Welch.
A 20-year statute of limitation for murder means there will be no punishment for the first four killings, including Welch.
Sentences will be passed later this week.
Reign of terror
For years, the group staged rocket attacks, bombings, shootings and bank robberies in central Athens and taunted authorities in letters to the media.
They eluded authorities until a botched bombing attempt last year led to the first capture and set off a hunt that led to all the arrests.
Defendants also included a religious icon painter, four sons of a Greek Orthodox priest and a woman.
"Greek justice spoke today. Its decisions are respected by all"
November 17 was born in the era of 1960s-1970s radicalism that also spawned Germany's Baader-Meinhof gang and Italy's Red Brigades, groups that have now mainly disappeared into history.
Greece does not have the death penalty and some convicted murderers - unless they get multiple sentences - could be freed after about 20 years.
Among those in court for Monday's verdicts was Athens Mayor and Olympic Games host Dora Bakoyiannis, whose parliamentarian husband Pavlos Bakoyiannis was gunned down by the gang in 1989.
"Greek justice spoke today. Its decisions are respected by all," said Bakoyiannis.