In the early days of the First World War on 5 August 1914, Monro was deployed to France as General Officer Commanding 2nd Division, which played an important part in the First Battle of Ypres. He led with what a subordinate described as "the gift of personal magnetism". On 27 December 1914 he became General Officer Commanding I Corps, with the temporary rank of lieutenant-general. He was made General Officer Commanding Third Army on 15 July 1915 with the temporary rank of general. He was promoted to the permanent rank of lieutenant-general on 28 October.