The judge in the southwestern city of Bordeaux who sentenced 48-year-old Thierry Tilly described him as the perpetrator of a "machiavellian plot" against Ghislaine de Vedrines and 10 members of her family.
The family remained under Tilly's control from 2000 to 2009, shutting themselves away in their home, selling their belongings, including the family chateau, and handing the guru 4.5 million euros.
Tilly, who had persuaded the family that they had fallen victim to an international plot only he could deal with, was convicted of psychological abuse.
The fraudster, who nicknamed himself Oxford, after the British city where he had his base, was also stripped of his civic rights for five years.
French tax authorities eventually took possession of the chateau when the family failed to pay taxes, after which Tilly took them to Oxford to live.
Tilly was arrested in Switzerland in 2009 after a family member, who successfully escaped his clutches in Oxford, launched a complaint.
His accomplice, Jacques Gonzalez, will serve four years in jail.