The 17-year-old midfielder has clearly made an impression on manager Erik ten Hag, who has included him in a number of Premier League match day squads, although he is yet to play in the top flight, sitting on the bench six times.
Mainoo has played in cup competitions, though, starting in a Carabao Cup victory over Charlton last month and coming on in the FA Cup win over Reading.
The England Under-18 international has now extended his stay with Manchester United, although the length of the contract was not confirmed.
Speaking after his debut against Charlton, when he played 60 minutes before being replaced by Casemiro, Mainoo was thrilled and desperate for more first team chances.
‘It is an unbelievable feeling, I have been at this club all my life and it’s all been building towards tonight. To have my family here watching me, it’s been amazing,’ he told MUTV after making his debut in the Carabao Cup.
‘Of course there were a few butterflies but it was exciting. I played at Old Trafford in the Youth Cup and I remember that feeling, it was an amazing feeling. So to have that again, it is always brilliant.
‘It was a tough, physical game but I believe in the lads and thought we should always come out on top.’
He added: ‘I have been in training a few times with the lads and they have all made me feel welcome, they have all given me words of advice and encouragement. I have felt at home.
‘To stay around and get more games is the ultimate goal, I have been at this club my whole life so that is the ultimate goal.’
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