Callum was born in London and grew up on a Chelsea estate - he is a huge Chelsea football fan - where, he claimed in a 2014 interview, "I learned more from films than I did through life itself. Six foot two tall he decided to go into acting at age eighteen taking some courses and working as a model for Burberry, fronting their 2011 campaign photographed by Mario Testino. Whilst continuing to take acting lessons he was cast by John Boorman to play the director's younger self in 'Queen and Country', the sequel to 'Hope and Glory', in 2012 though it was over a year before the project got off the ground. In the meantime he appeared opposite Helen McCrory in 'Leaving'. a television drama about a middle-aged woman's affair with a younger man. On the strength of this and the eventual release of 'Queen and Country' Screen International declared him one of its Stars of Tomorrow in 2014. In 2014 he also appeared in Channel 4's drama serial 'Glue', as one of a group of youngsters investigating the death of Callum's brother in a community of travellers and in early 2016 he could be seen as part of the huge ensemble cast in the BBC TV adaptation of 'War and Peace.'