Harry Maguire had agreed a deal with Manchester City before rivals United offered him more money this summer.
The 26-year-old moved to Old Trafford in an £80million switch from Leicester last week and signed a contract worth up to £200,000 a week.
The England international put pen to paper on a huge six-year deal with the option for a further 12 months.
Maguire supported United as a child and his allegiance to his boyhood club proved to be enough to sway his decision.