Johnston fought in the First World War, serving on the frontlines of the Western Front with the original Expeditionary Force, being wounded four times and rising to the rank of Brigadier General. Initially Johnston served as signals officer for 7 Infantry Brigade. He went on to serve in that capacity with 3rd Division before becoming, in turn, Brigade Major, Commanding Officer 10th Battalion Cheshire Regiment and finally Officer Commanding 126 Infantry Brigade. Johnston took part in the engagements at Mons, Le Cateau and the subsequent retreat and advance to the Aisne. In 1916 whilst commanding the 10th Battalion Cheshire Regiment he was award the Distinguished Service Order (DSO) for his actions at the Battle of the Somme. A year later in 1917 he was to be awarded a bar to his DSO for his actions at Ypres. During the course of the war he was Mentioned in Despatches no less than five times and in 1916 was awarded the French Croix de guerre. Johnston wrote a diary chronicling his activities in the war.