Mendy becomes Lyon's second signing of the week after the club wrapped up the €10million transfer of Chelsea forward Bertrand Traore, who spent last season on loan at Europa League finalists Ajax, on Monday.
Left-back Mendy has signed a five-year deal at Lyon and he was unveiled at a news conference on Thursday, where club president Jean-Michel Aulas said Ghezzal – who came through Lyon's youth ranks – will not be signing a new deal at the club, while Gonalons is set to join Roma.
"I'm very happy to be at Lyon and I cannot wait for the season to start," Mendy told reporters. "It is a club that has ambition and a great project. I did not hesitate.
"There are many young players at Lyon. I was told that it was a good group, a good collective."
Aulas also confirmed Lyon have agreed to let 28-year-old midfielder Gonalons move to Roma.
"Maxime came this morning and signed his endorsement of termination," Aulas said. "We started talks with Roma. We left on good terms."