The centre-back starred for his country at the World Cup, scoring in the quarter-final win over Sweden, and was linked with a big-money move to Old Trafford as United sought defensive reinforcements in the close season.
But Maguire remained with Leicester, saying he respected the club's decision not to sell him, and he has now committed his future to the 2015-16 Premier League champions.
"Ever since I came to this club, I've always reiterated how good they've been to me," Maguire told the club's website after extending his contract through to 2023.
"They gave me an opportunity to play in the Premier League and built a great platform for me to go and perform at the World Cup, so I thank them a lot. I owe them a lot and I'm looking forward to the future."
Maguire was named Leicester's Player of the Year after appearing in all 38 league matches last term, having signed from relegated Hull City in June 2017.
The 25-year-old made his England debut in a World Cup qualifier in Lithuania last October and established himself as a regular ahead of the finals in Russia.
He has been tipped to shine "for any top club" by team-mate and former United player Jonny Evans, who added: “Physically he’s got it and he’s good with the ball. His technique is fantastic and I think he proved that throughout the World Cup."
Meanwhile, Foxes goalkeeper Kasper Schmeichel is confident Maguire will develop into "one of the best defenders in the world" over the next 10 years.
“He’s got absolutely every attribute you could possibly want in a defender," he said. "He’s an exceptional football player, very intelligent, he can see a pass and pick it out, positionally he’s very astute and very clever."