 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.