Garnacho arrived at Old Trafford from Atletico Madrid last year, with United paying a £419,000 fee to obtain the teenage winger.
The 17-year-old enjoyed a great debut campaign for the Under-18s, scoring seven goals and recording a further three assists in 17 appearances, including a hat-trick against Stoke City.
Garnacho has been rewarded for his fine start to life in England with his first professional contract.
The Spaniard declared his new deal in an Instagram post and the caption read: 
“It’s a dream come true to sign my first professional contract with @manchesterunited.
“Proud day for me and my family, thanks to everyone who has helped me get to this moment,” accompanied with a love heart emoji.
While the versatile forward still remains behind the likes of Anthony Elanga, Joe Hugill and Shola Shoretire in the queue for a spot in Ole Gunnar Solskjaer’s first team, he will be hoping that another big season will put him in the manager’s thoughts.
United have continued to strengthen their academy ranks this summer with the signing of midfielder Ethan Ennis after the 16-year-old turned down a contract offer at Liverpool.