The 20-year-old was recalled to Thomas Tuchel’s England squad last month after a resurgence under interim manager Michael Carrick since the start of the year, which has seen him flourish.
Mainoo, who was on wages of £25,000-a-week, will now take home approximately £120,000-a-week according to reports.
The midfielder has missed just one of Carrick's 13 games in charge, playing all but 14 minutes in the other 12 fixtures as he grows back into the side after being frozen out by former manager Ruben Amorim.
The club revealed that he has put pen to paper on Thursday afternoon as the youngster spoke on his ambitions, especially as the side are on the verge of qualifying for the Champions League.
“Manchester United has always been my home; this special club means everything to my family.
“I have grown up seeing the impact that our club has on our city, and I relish the responsibility that comes with wearing this shirt.
“From The Cliff, to Littleton Road, to Carrington and ultimately Old Trafford, the journey so far has been incredible. I have the privilege of living my dream every single day, with the same relentless desire to succeed here as when I joined my first training session at the age of six.
“We can all feel the momentum building inside the club. I am determined to step up and play my role in helping Manchester United to regularly fight for major trophies in the years ahead.”
The England midfielder had been in negotiations with club bosses for the last three months and the new deal will come as a relief to fans who recently saw Harry Maguire commit to a one-year extension earlier this month.