Greenwood, 18, is a product of the club’s famed Academy and has been at Manchester United since the age of seven.
The England Under-21 international became Manchester United’s youngest starter in the Premier League era when Ole Gunnar Solskjaer handed him his full debut against Cardiff City in May.
Mason Greenwood said:
“Having grown up as a Manchester United fan, playing for the first team is a real dream come true. I know that, when I play for this club, I represent the whole Academy and I want to repay the faith that my coaches have shown in me with my performances on the pitch.