The 33-year-old was due to become a free agent at the end of the campaign, but his form under interim manager Michael Carrick — who replaced Ruben Amorim in January — has earned him a new deal and a remarkable late-career renaissance.
Maguire has started every Premier League game since Carrick took charge, helping United climb from sixth to third in the table and back into Champions League contention. He has won 78 percent of his aerial duels this season and was man of the match in a 3-2 win over Arsenal.
The turnaround earned him a first England recall in 18 months for Thomas Tuchel's March friendlies against Uruguay and Japan. He started against Uruguay and impressed, with Tuchel describing him as "very good on the ball, very calm, strong in the air and a weapon for set-pieces."
Maguire also appeared off the bench in the Japan defeat, a game that highlighted England's defensive frailty without him. His World Cup place remains uncertain — Tuchel has indicated Marc Guehi and Ezri Konsa are currently ahead — but Maguire's form makes him impossible to ignore ahead of the summer tournament in the United States.
A veteran of two World Cups, Maguire was outstanding in Russia in 2018 as England reached the semi-finals and started every game in Qatar four years later.
"Representing Manchester United is the ultimate honour," said Maguire. "I am confident that our best moments together remain ahead of us."
Director of football Jason Wilcox added: "Harry represents the mentality and resilience required to perform for Manchester United."