The proposed deal, which runs until 2031, is worth 120,000 pounds per week and would see the 21-year-old quadruple his current wages.
The breakthrough follows a turbulent period for Mainoo, who only months ago had been considering a temporary exit from Old Trafford.
Frustrated by a lack of opportunities under former manager Ruben Amorim, he had explored a six-month loan move to Napoli that would have taken him through to the end of the season.
Concerns over limited playing time even led to fears it could jeopardise his chances of making England's World Cup squad.
However, circumstances shifted dramatically after Amorim's dismissal and the appointment of Michael Carrick as interim manager. Carrick quickly reinstated Mainoo into the side, a decision that has paid dividends.
The young midfielder has since become an influential figure, playing a key role in United's climb to third in the league table.
Although Carrick typically steers clear of contract negotiations, he made an exception in Mainoo's case. The interim boss strongly advocated for the midfielder behind the scenes, emphasising both his current impact and long-term value to club executives.
Negotiations between Mainoo and United officials have been ongoing for around three months, culminating in an agreement that reflects his elevated status.