The Premier League champion has finally sealed a deal for the France international, having made acquiring an elite defender to partner with Vincent Kompany a summer priority.
The France center back was in Manchester in July to complete a medical after the two clubs agreed to a 40 million euro deal.
City had tried to sign Mangala during the January transfer window but the Portuguese club held firm, despite the investment trust that owns part of his economic rights willing to do a deal.
The Premier League champion also unsuccessfully attempted to sign Mangala's teammate, Brazilian midfielder Fernando, in the winter window, but announced his signing during the World Cup.
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“City is a top club in Europe," Mangala told the team's official website. "For me, it was an important step to leave Porto and join Manchester City in order to continue my progress. I want to win titles and I believe I can do this. I am ambitious and this is why I am here.
“I’m very happy to come to England because for me, the Premier League is the best league in the world. It is a very intense and very aggressive competition. There are plenty of goals, so it is also really nice to watch and I can’t wait to get started."
City had prioritized the capture of an elite central defender to partner with Vincent Kompany, and the 23-year-old Frenchman becomes the club's sixth summer signing after Bacary Sagna, Fernando, Willy Caballero, Frank Lampard and Bruno Zuculini.