Mason joins the likes of Jonny Wilkinson, Gavin Henson and Matt Giteau at Toulon, after cutting short his spell with Super League outfit Hull KR.
The 31-year-old Australian international forward, who courted controversy throughout a chequered and colourful career in the 13-man code, made only six appearances for Rovers before being de-registered after failing to obtain a Tongan passport which would have made him a non-overseas player.
Hull KR then stopped paying his wages after discovering he had gone behind their backs to talk to Toulon owner Mourad Boudjellal during a furtive trip to France and eventually opted to cut their losses by releasing him from his three-year contract.
Mason refused a loan move to Super League rivals Bradford in the hope of following in the footsteps of his former Canterbury Bulldogs team-mate Sonny Bill Williams, who made a successful transition from a rugby league back row forward into a strong-running centre in the 15-man game.
Although he has never played rugby union, Mason revealed on Sunday that he is set to play for the Barbarians against England at Twickenham next Sunday and against Wales at the Millennium Stadium on Saturday, June 4.
The Barbarians squad is expected to be announced later on Monday.
"I'm just so excited to be playing rugby and I'm looking forward to the challenge," he told the Sydney Morning Herald.
"I've certainly jumped in the deep end in agreeing to play for the Barbarians without having any rugby experience.
"If you ask all the boys, I've always told them I've wanted to have a crack at rugby and I'm ready to try the best I can to make an instant switch.
"I've just got to learn how to play rugby in a hurry and how to speak French because I don't know a word yet and they say it's not that easy to pick up."
Mason, thought to have been promised £346,000 a year by Toulon, has signed a 12-month contract, with a second year dependent on him starting 15 matches next season.
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