Born in Maidstone, Alex was formerly a member of the Sale Sharks academy before joining Saints Academy in 2015. 

Mitchell has represented England at international level, playing at Under-16, Under-17 and Under-18 levels, before being called up to the Under-20 squad in October 2015 alongside Northampton teammate Harry Mallinder.

Alex was then selected to play for the England Under-20s for the 2017 Six Nations campaign and featured on home soil as they hosted Scotland at Franklin's Gardens. 

The scrum-half was named among the England Under-20s squad again for the 2017 World Rugby Under-20s Championships in Georgia, and scored a try in each of their two opening pool games before starting in the semi-final against South Africa.

His debut season with the first team earned him nine caps as he featured in games against Leicester Tigers, Wasps and Saracens.

Mitchell also helped the Wanderers lift the Premiership Rugby A League trophy in 2017 and 2018, defeating Gloucester United and Exeter Braves respectively to bring the trophy to Northampton.