Greenwood, 19, has scored 21 goals in 82 games since graduating from United's academy and made his senior debut for England in 2020. He only signed a contract extension in October 2019 but the club decided to renegotiate his terms early to reflect his growing importance in Ole Gunnar Solskjaer's team.
"Mason is a fantastic talent and yet another fine example of the type of player that our academy produces," Solskjaer said. "He has made great progress this season, showing real maturity and adaptability to his game, which has seen him develop into one of the best young players in the country.
"Whilst it is easy for people outside of the club to forget that Mason is just 19, it is our job as coaches to ensure he reaches his phenomenal potential and every step we take is to help him along that path.
"Mason is naturally gifted and has a brilliant attitude; we know how important he is going to be for Manchester United over the coming seasons."
Greenwood added: "When you join the club at seven years old, you just dream of playing for the first-team one day. I've worked so hard to reach this level and the last two years have been amazing. There is so much that I want to achieve in the game and I know that this is the perfect environment to play my football.
"With the support from the manager, his coaching staff and so many great players to learn from; I know that I am developing every single day. There is a lot more to come from myself and this team and I am so committed to working hard week in, week out to help the group achieve success.
"I want to repay the club for the support that everyone has given me over the years and show everyone just what I am capable of."