The scrum-half has signed his first senior contract at Northampton, keeping him at the club until 2021.
Mitchell has been a part of the Saints Academy since arriving at Franklin’s Gardens from Sale Sharks in 2015.
After earning nine appearances in his debut campaign with the first team last season, Mitchell has already trumped that total this term with 12 outings.
The 21-year-old also has three tries to his name already this season, and he is now hoping to add many more to that tally after signing his new deal.
“I’m absolutely certain that Northampton is the best place for me to grow as a player, so I had no hesitation in signing a new contract here,” said Mitchell.
“It’s fantastic to have continued to build my first-team experience with Saints this year – we’ve got a brilliant environment here at the club and it’s been made clear that if you’re good enough, you’ll be given opportunities to impress.
“I’m still developing on the field and know I have a lot to learn yet from the coaches and senior members of our squad, but I’m excited to keep working hard and wear the black, green and gold jersey as often as I can.”
Born in Maidstone, Mitchell represented England from Under-16 to Under-20 level, playing in the Six Nations campaign and World Rugby Under-20s Championship in 2017.
He also helped the Wanderers to lift the Premiership Rugby A League trophy in 2017 and 2018.
But it is in first-team colours that Mitchell has really impressed in recent months, with the youngster starring across the Gallagher Premiership, European Challenge Cup and Premiership Rugby Cup competitions.
And Saints boss Chris Boyd said: “Alex has really stepped up to the mark this season so far and is providing excellent competition for places within our half-backs.
“I’m looking forward to seeing him continue to progress – he has bags of potential and is a popular member of the squad.
“Alex’s best years are definitely still ahead of him and it’s fantastic for us that he shares our vision for the future at Saints.”