Mainoo, 17, was a key part of the recent FA Youth Cup success and is regarded as one of the most exciting prospects in the club's academy set-up.
The Stockport-born central midfielder has been a part of the United youth set-up since the age of nine.
Mainoo posted to social media: "Happy to sign my first professional contract with @manchesterunited great way to end a successful season."
Mainoo started every match as United went on to win the Youth Cup against Nottingham Forest last week, where he excelled with a combative performance at the heart of Travis Binnion's side.
The 17-year-old made 31 appearances in total for United at academy level this season, even appearing for the U23 side when he was just 16 and also playing in the UEFA Youth League.
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