Corbett was 18 when the First World War broke out; he secured a Commission in the Ayrshire Yeomanry and went with them to Gallipoli in October 1915. The Ayrshires were amongst the last troops to be evacuated and went from there to Egypt, forming part of the cavalry reserve in the Second Battle of Gaza. He then transferred to the Grenadier Guards; after re-training he was sent to the Western Front in March 1918, joining the 1st Battalion at Mercatel as a lieutenant. This coincided with the German Spring Offensive (Operation Michael), and the battalion was soon in action. At Boyelles on 30 March 1918, Corbett attempted to dig out some wounded soldiers who had been buried by artillery, while "under heavy fire and in full view of the enemy" and was awarded the Military Cross but received a leg wound which left him with a permanent disability.