The 24-year-old Frenchman has agreed a four-year contract and will become Leicester's third signing since winning the title after goalkeeper Ron-Robert Zieler and defender Luis Hernandez.
Mendy played for Leicester manager Claudio Ranieri during a spell with Monaco.
“The team have just enjoyed a great season and it’s excellent to be here. For me I want to play at this level. I like the Premier League and I follow every match. It’s fast and physical and I want to impose myself on the league," Mendy told the club website.
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“I know the coach, I know his mentality and I know his philosophy. When he speaks it’s direct and clear and for me I like this."
(Reporting by Ed Osmond, editing by Pritha Sarkar)
