Born at Port Patrick, Scotland, William Milroy was a farm servant at the time of his enlistment,aged 20-1/2 years, with the Argyll and Sutherland Highlanders (1st Battalion) on 25 June 1896 at Aldershot - service no 5788. He served as a Private and Corporal during Anglo Boer War: Modder River, Paardeberg, Driefontein, Transvaal to 1902. He was discharged at Johannesburg on 25 March 1903 on payment of 18 pounds (according to King's Regulations), having served 6 years and 374 days with the Regiment.

William Milroy then served as a Constable with the Central South African Railways for a year (20/3/1903-9/6/1904); as a fire attendant for the Johannesburg Municipality for 1 month (July 1904) and then with the Transvaal Town Police from 10 August 1904 to 30 June 1908.

About this time Milroy migrated to New Zealand. In 1912 he joined the New Zealand Permanent Staff, at Rangiora, and was living in Blenheim when he married Mary Ann Reid Stalker on 27 April 1916. At the time described himself as a 40 year old bachelor, occupation 'Soldier'. Just over a year later William Milroy joined up with the NZEF, service no 66233 WO2 Staff Sergeant William Milroy. He served for 1 year, 227 days, mainly as part of the 'Egyptian Expeditionary Force', and remained with the NZ Permanent Staff following his discharge from the 1NZEF.

He remained with the NZ Permanent Staff until 31 March 1931 when he was discharged as medically unfit for further service. At the time of his death, three years later on 6 August 1934, William Milroy was living in Wellington. His occupation was listed as chimney sweep (Sergt. Major). He was survived by his wife and a sixteen-year old son.