The 25-year-old joins the Premier League champions on six-year contract and will don the number 10 shirt left vacant by Sergio Aguero.
Grealish after signing the new deal with City said;
"It was obviously a difficult few months because I have been an Aston Villa fan for my whole life, but when I spoke to the manager here and you see what type of players they have got here, in the end it was something I couldn't say no to.
"There are obviously a lot of reasons why I decided to come here. The manager has been a massive factor; playing under the best manager in the world was something I couldn't really turn down. Obviously the players that we have got here, I think everyone knows how much I admire Kevin [De Bruyne].
"But competing for major trophies is something that I wanted to do. It was something I couldn't turn down."
Man City director of football Txiki Begiristain said of the England international.
"Jack's development over the past few seasons both for club and country has been plain for everybody to see.
"His natural talent together with his commitment to improve as a player, has seen him become one of the most exciting attacking players in world football today."
Grealish will be available for selection when Man City play Leicester City in Saturday's Community Shield against Leicester at Wembley.
Having graduated through the youth ranks of Villa academy, Grealish has featured over 200 appearances for the club, with 32 goals.
