Born 17 August 1864 at Digby, Nova Scotia, the son of Alexander Macnab, civil engineer. He was educated at Winchester and received his medical training at King's College Hospital, where he served as house surgeon after taking the Conjoint qualification in 1887. At Netley, where he trained for the Indian Medical Service, he won the Montefiore gold medal in military surgery and was gazetted surgeon, IMS, on 31 March 1890, and commissioned as medical officer to the Corps of Guides. He saw active service on the North-West Frontier in 1891, winning the medal and clasp in the night attack on Ghazikot in the Hazara expedition; he won the medal and clasp of the Chitral expedition at the relief of Chitral in 1895. He took the Fellowship in 1896. In the Malakand expedition of 1897-98 he won two clasps, when he took part in the defence and relief of Malakand, the relief of Chakdara, the action at Landakai, and operations in the Mahmund country and at Utman Khel and Buner. He was promoted major on 31 March 1902, and served with the Indian Expeditionary Force in Somaliland in 1903-04; during 1904-05 he was surgeon to the Viceroy, and then for three years a civil surgeon at Simla. From 1909 till the out¬break of war he was Residency surgeon in Kashmir. He was promoted lieutenant-colonel on 31 March 1910. During the war of 1914-19 he served as medical officer in command of the hospital ship *Syria*, was on active service in France and Belgium in the summer of 1916 and the autumn of 1917, and with the Egyptian Expeditionary Force in Egypt, Palestine, and Syria through 1918 as ADMS of a division. He was three times mentioned in despatches and created CB 1918 and CMG 1919. He was promoted brevet colonel on 14 January 1916 and colonel on 2 April 1919, and served as DDMS at Army headquarters in India 1920-21, retiring on 15 August 1921. Macnab married in 1895 Honoria, daughter of Lieutenant-General Sir Lewis Dening, KCB DSO; she died in 1942; there were three children of the marriage, a son and two daughters. He died at Hedges, Park Road, Winchester on 20 May 1947, aged 82, and was cremated after funeral service at Hyde Parish Church.