Greenwood, 19, signed a new deal on Tuesday to remain at United until at least 2025, with the academy graduate recognised as one of the most promising talents of his generation.
Greenwood has scored 21 goals and 82 appearances for United, and though he has found this season tougher than his breakthrough campaign last term, the teenager says there is plenty more to come.
“When you join the club at seven years old you just dream of playing for the first-team one day,” Greenwood said.
“I’ve worked so hard to reach this level and the last two years have been amazing. I want to thank the academy for everything they have done for me and for their continued support to this day.
“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, Kieran (McKenna) and the rest of the 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 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.”
United manager Ole Gunnar Solskjaer, who is a big fan of Greenwood’s, added: “Mason is a fantastic talent and yet another fine example of the type of player that our academy produces.
“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.”
Additional reporting by PA.