The 21-year-old forward will leave the club at the end of an injury-hit season which saw a loan move to Salford City terminated early in January.
Mellor was then set to join Crawley Town on loan before deadline day but saw a deal collapse because of paperwork issues between the two clubs. The versatile forward joined United at the age of eight and made his senior debut for the side in the Europa League defeat away to Astana back in November 2019.
However, his progress was hampered the following summer when he suffered a serious knee injury that left him sidelined for the entire 2020/21 campaign.
Mellor posted to social media: "The end of my first chapter in football.
"Making a debut for Manchester United is something that only a small number of people have the honour to say that they have achieved.
"I've been extremely fortunate to represent this Club for over 10 years.
"There are so many people that I have to thank for their support along the way. Many have become my closest friends throughout my time here. I'll always be connected with this amazing Club.
"Now, a new chapter starts. I'm ready for that next step and I'm excited to show everyone what I can do."
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