The Red Devils announced on Friday that a deal had been agreed with the 2015/2016 Premier League champions to take Maguire to the Theatre of dreams.
Maguire moves to Old Trafford on a six-year deal with the option for a further year.
Maguire’s transfer fee surpasses the £75m Liverpool paid Southampton to price Virgil van Dijk from the Saint Mary’s stadium in January 2018.
"I am delighted to have signed for this great club," Maguire said after completing his move to United.
"I have thoroughly enjoyed my time at Leicester and would like to thank everyone at the club, and the fans, for their fantastic support over the past two seasons. However, when Manchester United come knocking on your door, it is an incredible opportunity.
"From my conversations with the manager, I am excited about the vision and plans he has for the team. It's clear to see that Ole is building a team to win trophies. I am now looking forward to meeting my new team-mates and getting the season started."
The move completes a rapid rise for Maguire, who suffered relegation with Hull City just two seasons ago before joining Leicester City and starring for England at last summer’s FIFA World Cup.
Ole Gunnar Solskjaer had been looking for a dominant centre-half as the club look to improve upon last season’s disappointing sixth-place finish.
Maguire is Ole Gunnar Solskjaer’s third summer signing following the captures of Aaron Wan-Bissaka and Daniel James from Crystal Palace and Swansea City respectively.
The 26-year-old will be available to make his debut for United in their Premier League opener against Chelsea at Old Trafford on Sunday.
