Armando, 56, was sentenced to 18 years in jail for his role in the 1992 robbery of the Bank of France in Toulon when raiders got away with 146 million francs (€22 million today), only around a tenth of which was ever recovered.
In April, he was arrested in the Netherlands under a European arrest warrant in connection with drug trafficking and was handed over to French authorities on Friday.
Armando was found hanged in his cell within hours of arriving at a detention centre in Marseille on Friday evening, sources said.
An investigation into the death is under way.
Armando was thought to have been the brains behind the Toulon bank robbery, known as "the heist of the century" in France.
He was due to appear before magistrates on Saturday in connection with a large seizure of cocaine in the port of Rotterdam. The illegal cargo was allegedly attached to a petrol tanker.