The Inter Milan striker flew in for a medical in the morning and City confirmed later that he had signed a five-year deal.
City coach Roberto Mancini also surprisingly named Robinho in his 25-player squad, saying he would sit down with the unsettled Brazil striker next week to thrash out his future.
James Milner, Aston Villa's England winger, is also expected to join City in the next few days with Craig Bellamy and midfielder Stephen Ireland on the way out of Eastlands.
"I think that Mario is one of the best players of his age in Europe, and I am very happy to be working with him again," Mancini told the club's website.
"His style of play will suit the Premier League and because he is still so young there is a big chance for him to improve. He is a strong and exciting player and City fans will enjoy watching him."
This season the Premier League has imposed a 25-man limit on squads, with eight "home-grown" players included in that number. That is putting extra pressure on coaches who are being forced to let some players leave.
Neither Bellamy, one of City's most effective playters last season when they narrowly missed a top-four finish, or Ireland are in Mancini's 25-man list.
"He only arrived back yesterday and is tired from the World Cup and other games he's been playing," said Mancini. "But he's on my list and I will talk to him next week about the situation."
One player definitely leaving is Wales striker Bellamy, who said last week he had not spoken to Mancini for six months and knew he was not part of the Italian's plans.
"This is the problem of the list," said City's manager. "There are players on there and those who aren't are out.
"Craig is not on the list. What he said isn't important, it's false. I work with the players every day and speak with them.
"I don't want to talk ab
