Delamain was born in Saint Helier, the son of Charles Henry Delamain and Susan Sarah Christina Gun. He attended the Royal Military College and was commissioned as a lieutenant in the Princess Charlotte of Wales's on 22 October 1881. On 13 January 1885 he was seconded for service with the Indian Staff Corps, and was commissioned as a lieutenant in the Bombay Staff Corps on 1 February 1885, with seniority of 22 October 1881. He was promoted to captain, 22 October 1892, and given the temporary rank of Major, 4 November 1898. As Commandant of the Native Military Base Depot during the Boxer Rebellion he was mentioned in despatches. On 26 May 1901 he was appointed a Special Service Officer on the staff of the China Field Force, and his rank of major was made substantive on 10 July 1901. In October 1902 he was in command of a detachment from the 23rd Bombay Rifles sent via Aden to British Somaliland as reinforcement during the Somaliland Campaign. In 1905 he was again mentioned in despatches for his service with the Outram's Rifles, commanding the escort to the Aden Boundary Commission for eight months, was awarded the Distinguished Service Order on 14 April, and promoted to lieutenant-colonel, with seniority of 12 January 1905. On 12 January 1908 he was made brevet colonel, and substantive colonel on 1 January 1911. He was appointed assistant adjutant-general on 17 November 1912.
Delamain was appointed temporary brigadier-general, 27 September 1914. He commanded the 16th and 17th Brigades at the Battle of Es Sinn. His conduct was described as "admirable" in General Nixon's despatch, and Delamain was promoted to major-general, 29 October 1915, for distinguished service in the field. He became a prisoner of war on 29 April 1916.
Post-war
On 14 November 1919 Delamain was given command of a division, and he was promoted to lieutenant-general on 1 April 1920, with seniority later backdated to 21 December 1919. He served as Adjutant-General, India from 10 November 1920 to 28 March 1923, when he retired from the Indian Army. Delamain was also colonel of the 117th Mahrattas from 28 October 1921 until he reached the age limit, being replaced by Major-General R.C. Wilson on 18 February 1932.
