The elegant Dumas showed no sign of emotion as the judge convicted him of receiving illegal funds from oil giant Elf Aquitaine. He was given an additional two-year suspended jail term and ordered to pay a fine of one million francs ($130,400).
A close friend of the late Socialist President Francois Mitterrand, the 78-year-old Dumas had denied the charges and his lawyers immediately said they would appeal the verdict.
The high-profile trial opened in January and gripped the public imagination with its allegations of greed, sex and ambition that laid bare accusations of endemic corruption during the twilight years of the Mitterrand era.
The panel of three judges also convicted four of Dumas' six co-defendants on charges ranging from embezzlement to receiving misappropriated funds from Elf between 1989 and 1993.
Dumas' one-time lover, Christine Deviers-Joncour, received three years in prison, of which 18 months were suspended, and was fined 1.5 million francs.
Former Elf chairman Loik Le Floch-Prigent was handed a three-and-a-half year prison term and a 2 million franc fine, while his No. 2 at the company, Alfred Sirven, received four years in prison and a 2 million franc fine.
Wednesday's verdict is a devastating blow for Dumas, considered one of the sharpest minds of his generation a Resistance fighter during World War II who rose through political ranks to become a leading socialist.
He was the head of France's Constitutional Court the top legal position in the country until his resignation last year.
Magistrates accused Dumas of obtaining a fictitious job for his mistress at Elf in 1989 when he was foreign minister, and then benefiting from her 64.5 million franc pay-out in the shape of luxury gifts worth a total 800,000 francs, including a $1,500 pair of boots and $40,000 worth of antique statues.
Dumas denied knowing the gifts were aimed at winning his support for the multimillion-dollar sale to Taiwan's navy of six frigates built by a French state-owned firm.
The frigate sale is the subject of a separate inquiry, although during the Dumas trial, Deviers-Joncour testified that she received $6.4 million from a slush fund at Elf in return for her efforts to persuade Dumas to support the sale of the frigates.
Judge Sophie Portier acquitted Dumas of being party to the original job scam, but said he had knowingly enjoyed the fruits of Deviers-Joncour's huge salary.
One of the highlights of the trial was supposed to be the eagerly awaited testimony of Sirven, who was portrayed as the point man for a slush fund at Elf. But Sirven refused to testify.
He fled France four years ago but was captured in February in the Philippines after an 11-month manhunt that led investigators through more than 20 ities, coconut groves, cattle ranches and even shrimp farms.
He was put on a flight to Germany, where authorities held him for four days in a futile attempt to question him about yet another scandal bribes allegedly paid in the sale to Elf of the Leuna oil refinery in the former communist East Germany.
Magistrates are continuing investigations into a raft of other graft cases involving politicians from all parties, but Dumas is the most senior figure to be convicted.
The so-called "affaire Dumas" represents only a tiny part of the whole Elf jigsaw puzzle.
Among the various strands still under investigation are charges that Elf gave slush funds to the political party of former German Chancellor Helmut Kohl.
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