Neither club has publicly commented on the transfer fee, but the deal is widely reported to be worth $97 million.
Maguire, who has made 20 international appearances for England, becomes Ole Gunnar Solskjaer’s third signing of the transfer window alongside winger Daniel James and right-back Aaron Wan-Bissaka.
The central defender has signed a six-year deal at United, which finished sixth in Solskjaer’s first season with the club.
“I am delighted to have signed for this great club,” said Maguire, who made 76 appearances for Leicester.
“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 teammates and getting the season started.”
The 26-year-old, who is strong in the air and good with the ball at his feet, will look to shore up a defense that shipped 54 goals in the Premier League last season — more than champions Manchester City and second-placed Liverpool combined.
“Harry is one of the best center backs in the game today and I am delighted we have secured his signature,” said Solskjaer.
“He is a great reader of the game and has a strong presence on the pitch, with the ability to remain calm under pressure — coupled with his composure on the ball and a huge presence in both boxes — I can see he will fit well into this group both on and off the pitch.”
United starts its new campaign at home to Chelsea this Sunday.