The 23-year-old, who joined the French champions from Marseille last summer, has agreed a deal reported to be worth a record £52 million (58 million euros, $67 million) for a defender.
The France player will link up with his new teammates in the United States where Pep Guardiola’s side are preparing for their second pre-season tour game against Real Madrid in Los Angeles.
“I am absolutely delighted to be joining Manchester City,” Mendy said. “They are one of Europe’s leading clubs and in Pep Guardiola they have a manager committed to playing attacking football.
“I am sure that over the next few years we will be successful.”
Tottenham’s Kyle Walker and goalkeeper Ederson are the other players to join a new-look City backline with Mendy’s Monaco team-mate Bernardo Silva coming in to bolster midfield.
Guardiola was desperate to add Mendy to his full-back options after releasing Gael Clichy, Bacary Sagna and Pablo Zabaleta this summer, but City’s original £44.5 million ($55.7 million, 49.7 million euros) offer was rejected.
The former Barcelona and Bayern Munich boss had already lured England right-back Walker to the Etihad Stadium in a deal which could be worth up to £54 million ($70.1 million, 60.1 million euros) and Monaco wanted a similar amount for Mendy.
The deal took a while to complete, but City’s director of football, Txiki Begiristain, was pleased to have secured their first-choice target.
“Benjamin has all the qualities we are looking for in a full-back. For such a young player, he has a wealth of top-level experience,” he said.
“He is undoubtedly one of the world’s best full-backs, our number-one target in this position. We are all delighted to have him here at Manchester City.
“I’m sure he will prove a fantastic addition to the squad.”
Mendy becomes City’s fifth major signing of the summer and takes their spending to more than £200m (223 million euros, $260 million).
The Premier League giants, who have already raided Monaco in the close season having signed Silva, on Sunday welcomed another full-back, Real Madrid’s Danilo.
Mendy made his France debut in a 3-1 World Cup qualifying victory over Luxembourg in March, and won Ligue 1 in his first season with Monaco.
Man City finished third in the Premier League last season behind champions Chelsea and Tottenham.
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