After leaving Macclesfield alongside Savage last year, Mendy became a mainstay at Forest Green, making 47 appearances as the Gloucestershire club finished seventh and saw their promotion bid end with a play-off quarter-final loss.
Forest Green say their captain’s transfer could rise to nearly £1m, which would be a club-record fee.
Hearts, who finished second in the Scottish Premiership last season, considered Mendy one of Forest Green’s standout performers.
The Edinburgh side describe him as tall, physical and versatile—qualities that helped Rovers reach the play-offs. Often used at centre-back, his ball-playing ability let him step into midfield when required.
Born in France, Mendy progressed through the lower tiers of English football with Thamesmead Town, Greenwich Borough, Whyteleafe, Ashford United, Dartford and VCD Athletic, spent a season in Sweden with Amal, then had four seasons at Macclesfield.