Mainoo has become pivotal for interim manager Michael Carrick since he arrived earlier this year, building a strong relationship in midfield with Bruno Fernandes and Casemiro.
The England star is hunting down Champions League football with the club he almost left under former manager Ruben Amorim after the pair publically fell out.
Under Amorim, he did not start a single Premier League match and his longest league appearance was just 45 minutes.
In the 11 games the 21-year-old midfielder has played under Carrick, United have won eight times, drawn twice, and lost just once.
However, after working his way back into the side, The Mirror report that after talks he is poised to sign a long-term contract worth £120,000-a-week until 2031.
The midfielder has started in 12 of the past 13 United games and the club are clearly keen on locking him down to a new long term deal which would see him quadruple his current wages.
Mainoo won back his place in the England squad last month and could feature at the World Cup under manager Thomas Tuchel who will have been one of the first to see the midfielder improve since the departure of Amorim.
If the youngster is set to stay for the next 5 years, United are likely to build a team around him, especially as they aim to replace Casemiro who will depart once the season is over.