Kearsey was promoted to captain on 12 March 1904, while the following year he was transferred to the 10th Royal Hussars. He then served in the Territorial Force as an adjutant from March 1908 to April 1911. With the outbreak of the First World War, Kearsey attended the Staff College, Camberley, and was promoted to major on 11 November 1914. At the start of the war, he was posted as part of the Embarkation Staff at Southampton, following which he proceeded to Belgium with the 1st Life Guards, 7th Cavalry Brigade. He took part in the First Battle of Ypres and saw action at Neuve-Chapelle. In 1915, he was attached to the staff of the 7th Cavalry Brigade, and August 1915 he was appointed Brigade Major with the 1/2nd South-Western Mounted Brigade, seeing action in Gallipoli. He later commanded the 5th Dorset Regiment for months, till May 1916. Following this, he was appointed GS01 with the 54th Division, Egyptian Expeditionary Force, seeing action at the first and Second Battle of Gaza, both of which ended in Ottoman victories. He later commanded 1/5th King's Own Scottish Borderers, seeing action at the Battle of El Teb, during the course of which he was wounded. At the time he was a temporary lieutenant colonel. For his services in the conflict, Kearsey was decorated by the Kingdom of Serbia with the Order of the Karađorđe's Star, 4th Class with Swords and the Order of the Nile. Transferring to the fledgling Royal Air Force, he commanded the Cadet Wing and was appointed staff officer, 1st grade on 28 December 1918. During his life, Kearsey published a number of his records including, "The War Records of the York and Lancaster Regiment, South Africa, 1900–1902,1903", which detailed his time with the York and Lancaster Regiment in the Boer War. He also published records of the campaign in Egypt and Palestine during the First World War. He later wrote a number of books analysing military strategy.