Mitchell's father (also called Colin) came from an Argyllshire fishing family. Mitchell (Snr) worked in a solicitor's office and for the MacBrayne ferry company before serving in the 10th Battalion Argyll and Sutherland Highlanders in World War I. Mitchell (Snr) achieved the rank of captain (commissioned 'in the field') and was awarded the Military Cross at the Second Battle of Ypres, but when the young Colin asked him how he would only say, 'Oh, shooting rabbits'. He was badly gassed in 1918. After the war, he worked in the City of London and married a Glaswegian woman (née Gilmour) whose father worked as a manager for the LMS Railway company. The couple took up residence in the South London suburb of Purley where they had two children – Colin and Henrietta. The family lived in a modest semi-detached house and Colin would attend services at the local Presbyterian Church wearing a kilt. Mitchell received his formal education at the Whitgift Grammar School in Croydon.
