After leaving the Gunners at 18 to join Swansea City, Mitchell re-signed for Arsenal this season after spells in the non-league.
The 21 year-old is now acting as a senior player for the club's U21s and is also part of the senior goalkeepers pool.
He told arsenal.com: "Leaving Arsenal was extremely tough. It was my first real experience of rejection. I'd been at Arsenal my whole life, and at 18 it's a lot to take in, especially being the only player in my group not offered a deal.
"I joined the Baller League with TBJZL's team. It was the biggest crowd I'd ever played in front of, with around 100,000 watching online and about 5,000 in the arena, and it was also on Sky Sports. It was unreal.
"Off the pitch, I'm in my second year of a Business and Football Management degree through the PFA. It fits well around training because lectures are in the evenings, and I'm usually home from training by three.
"I rejoined Arsenal this season. Coming back to the club I support has been an incredible feeling. Training with the first team and being around some of the best players in the world is unbelievable. Watching David Raya up close, you see exactly why he is so good.
"The standards he sets every day are top-level, and he is pushed by Inaki, the first-team goalkeeping coach, to be at his best."