Walter was educated in the medical school of the Royal College of Surgeons, Edinburgh, where he took medals in chemistry and in medical jurisprudence, and also honours in anatomy, surgery, and midwifery. He obtained the Scottish triple qualification in 1889. After qualifying he was appointed principal medical officer to the Imperial British East African Company in 1889, and when that chartered company’s territories became a protectorate in 1895 continued to hold the same post. During his service in East Africa he gained the African war medal and clasp for Witu in 1890, a clasp for Mwele in 1895, and East and Central Africa Queen’s medal with clasp for Uganda in 1897-98, the African general service King’s medal with clasp for Uganda in 1900, and the Imperial British East Africa service medal in 1902. He received a letter of thanks from the Admiralty for service in the Mazumi rebellion in 1895, was present at the bombardment of Zanzibar, and received a sword of honour with the thanks of H.H. Sultan Hamond in 1896, and the order of the Brilliant Star of Zanzibar in 1895. He was in medical charge of the Anglo-German Boundary Delimitation Commission of 1898, and was a corresponding member of the Royal Scottish Geographical Society. On the outbreak of the Great War he took a temporary commission in the R.A.M.C., and was stationed at Redford as the medical officer in charge of the troops of the Forth defences. He died of appendicitis in Craigleith Hospital, Edinburgh. He was the son of John and Catherine MacDonald of Spean Bridge, Inverness-shire; and the husband of Isabella Hill MacDonald of 39 Merchiston Crescent, Edinburgh.