The outbreak of the First World War in August 1914 put all plans of retirement on hold and Lomax was given command of the British Army's 1st Division as part of the Expeditionary Force being dispatched to France under the leadership of Sir General Sir John French. After taking part in the Battle of Mons in August 1914, Lomax commanded the Division through the First Battle of the Marne, and in the counter-attack on the German invasion of the West at the First Battle of the Aisne. His direction of operations was so accomplished that it has been said that he was "the best Divisional General in the early days of the war". On the 19 October 1914 he received notice that he was to be promoted to the rank of Lieutenant-General, and was marked to be given the command of a Corps when one next became available.