The youngster, who was at Ipswich Town, has confirmed his move on social media.
Mexes, who is the son of former Premier League forward Robert Earnshaw, will be a part of the club's academy.
The starlet posted on his Instagram account: "Very proud moment for me and my family to announce my signing for @manchesterunited.
"Thank you for everyone who has helped me on my journey so far. The work starts now #manchesterunited #signing #blessed."
Earnshaw added: "This is really really special. My incredible son Silva Mexes Tyler-Earnshaw who is known as Silva Mexes has signed for Manchester United academy.
"I am so so proud. He has worked hard & it's such a blessing to see him find his passion and make this journey. I love you son.
"I am your number one fan. I will support you in any way possible and I know you have the qualities Go be GREAT."