The 23-year-old France international has signed a six-year deal with the Club and will wear the number 22 shirt.
He will now join up with his new teammates in the USA where Pep Guardiola and his squad are preparing for their second pre-season tour game against Real Madrid in LA.
“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.”
City’s Director of Football, Txiki Begiristain, said: “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 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 began his career with Le Havre, where he played more than 50 league games, before joining Marseille in 2013.
He impressed at the Stade Velodrome, making 101 appearances in three seasons and establishing a reputation as one of the best full-backs in the French top flight.
Last summer, he joined Monaco on a five-year deal and his arrival saw a significant improvement in results for Leonardo Jardim’s side.
Mendy’s energy down the left was key to Monaco’s attacking style as they won the Ligue 1 title and made it to the semi-finals of both the Champions League and the Coupe de France, scoring 159 goals in all competitions.
Their European run saw them take some notable scalps, beating Tottenham twice in the group stage, before knocking City and Borussia Dortmund out in the knock-out phase.
Mendy’s performances were recognised in the end-of-season awards in France, with the left-back named in the Ligue 1 Team of the Year, and he was widely regarded as being one of the best attacking full-backs in the Champions League.
He made his senior international debut in March and already has four caps.
Everyone at Manchester City is pleased to welcome Benjamin to the Club.