The 29-year-old had been a long-term target for the Scottish side and has now signed a three-year contract at Tynecastle Park.
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Forest Green have confirmed that Mendy leaves for a potential club record fee of nearly £1 million "if all targets are met".
Forest Green manager Robbie Savage told his club website: "It's an unbelievable opportunity for Mendy.
"I could not stand in his way of progressing his career and bettering his and his family's life whilst getting a record fee received for the club.
"They've got a fantastic player and a fantastic person. I'm incredibly proud of him."