The 27-year-old has agreed a five-year contract with Kevin Keegan's side after reaching an agreement with Leeds to cancel his contract.
The former Charlton player, who spent last season on loan at Middlesbrough, winning a League Cup medal, told City's website: "It is a fantastic club with a manager who is passionate about his football."
Although he agreed terms last Friday, the deal could not be finalised because of minor details that Mills needed to resolve with his Leeds contract.
The former Charlton player, who spent last season on loan at Middlesbrough, winning a League Cup medal, told City's website: "It is a fantastic club with a manager who is passionate about his football."
Although he agreed terms last Friday, the deal could not be finalised because of minor details that Mills needed to resolve with his Leeds contract.