That has resulted in the full-back being given the chance to move on this summer, with Bradford giving the young Englishman a deal until 2020.
On his Twitter account, the 21-year-old said: "Manchester United has been my home, my family for the last 15 years. It is every boy's dream to play for this club and I can proudly say I have achieved this.
"I want to take this opportunity to thank everyone - from all the coaches that have helped me along the way, the staff, the many players I've played alongside while growing up but, most importantly, the fans that have showed constant support throughout my whole time here.
"I have so many unforgettable memories and experiences that I'll forever be grateful for, but I personally feel now is the right time for me to move on and to start the next chapter of my career. I wish everyone at the club the very best and again thank you."
Riley becomes Bradford's second new signing, with Tom Clare already joining the Bantams from Barnsley.