He was born at
Greenisland,
County Antrim on 4 January 1872, the son of Charles Johnstone, a farmer, and his wife Mary McCreavy. He studied at the
Belfast Academical Institution and
Queen's College, Belfast. On qualifying, Johnstone took positions at
Royal Victoria Hospital, Belfast, Queen's College, and as a pathology student in 1896. He was a medical student in London and Vienna. ==Notes==