A further 60 defendants were also found guilty of genocide, but only by a majority 2-1 ruling by the judges, who acquitted some but not all on several of the lesser charges.
Only one defendant was acquitted on all charges.
In Zimbabwe, where Mengistu has been living, officials ruled out the possibility of extraditing him to serve his sentence, describing him as "our special guest".
|"They all deserve death penalty"|
Judge Mehdin Kiros
"Despite the judicial ruling in Ethiopia, Comrade Mengistu still remains a special guest of the Zimbabwean government," Paul Mangwana, the information minister said.
"The government of Zimbabwe is fully aware of the judicial process that has taken place in Ethiopia and we respect it."
The trial related to atrocities committed during the 1977-78 "Red Terror" period when tens of thousands of people were killed or disappeared in Mengistu's bid to turn Ethiopia into a "workers' state".
He and his former top aides were also accused of the murders of former Ethiopian emperor Haile Selassie, whom they overthrew in a 1974 coup d'etat, and Orthodox Patriarch Abuna Tefelows.
Of the 73 accused, 14 have died and only 33 were present in court. Mengistu was among 25 defendants tried in absentia.
Only four of those present were handed prison sentences of between 23 and 25 years.