Ernest Harold Nellmapius was the son of Alois Hugo Nellmapius, industrialist and landowner in the South African Republic (Transvaal), and his wife Johanna Corlydia, born Hoffmann. He passed the matriculation examination of the University of the Cape of Good Hope in 1905. From 1906 he studied mining engineering at the South African College, Cape Town, passing the first examination in 1907 and the second in 1909. He then continued his studies at the South African School of Mines and Technology, Johannesburg, and was awarded both the Diploma in Mining Engineering and the degree Bachelor of Science (BSc) in Mining Engineering by the University of the Cape of Good Hope in 1911. That same year he became a member of the Geological Society of South Africa and was still a member in 1920. In 1912 he published a 'Note on the nature and origin of the Vredehoek tin deposits' in the society's Transactions.
In August 1913 Nellmapius married Frieda Hester Beck, daughter of Sir J H Meiring Beck*. They had two sons and two daughters. When his father-in-law died in 1919 Nellmapius bought from him the Drostdy at Tulbagh and settled there to farm until 1947. He was appointed as a member of the Tobacco Industry Control Board in 1944 and was re-appointed in 1952.