The 21-year-old, who celebrated his birthday on Sunday, has been in negotiations with the club for the past three months.
A verbal agreement has been reached and a formal announcement is expected before the end of the season, reports the Mirror.
Mainoo's revival this season has been the defining factor in the deal coming together. Under Ruben Amorim, the England midfielder made no Premier League starts and came close to joining Napoli on a six-month loan to preserve his chances of making Thomas Tuchel's World Cup squad.
When Amorim was sacked in January and Michael Carrick took over on an interim basis, Mainoo chose to stay — a decision that transformed his campaign.
Carrick restored him to a central role in the double pivot alongside Casemiro and he has started 12 of United's last 13 matches, with the club climbing to third in the Premier League.
The interim head coach spoke directly to United's hierarchy about the importance of extending Mainoo's contract, outlining his long-term value to the squad.
The new deal represents a significant financial step up from his current £20,000-per-week salary. With Casemiro departing in the summer and questions remaining over Bruno Fernandes's future,
Mainoo's central position in the next phase of United's midfield rebuild makes the renewal even more significant.