Franz S. Erni trained as a hotel keeper in France and England and came to Cape Town in 1899 to work at the Mount Nelson hotel. In March 1905 he married Katharina Helene Charlotte Wilhelmine Schwenn, with whom he had two sons. The next year he resigned his post and moved to German South West Africa (now Namibia) to become a storeman with the company that was building the railway line between Luderitz and Keetmanshoop. However, shortly after diamonds were discovered along the Namib coast in 1908 he joined a prospecting company and was involved in exploring the coastal desert. He also became a hotel keeper at Aus, and acquired a farm near the town. As a result of his work in the desert he developed an interest in its seed-bearing plants, particularly the succulents, and tended the plants he collected on his farm. From there he sent specimens to M.K. Dinter* and to the British Museum (Natural History). Some of his collecting was done in the company of Ernst J. Rusch*. The species Sarcocaulon ernii, Cephalophyllum ernii and Conophytum ernii were named after him, the first of these by Dinter, the second by H.M.L. Bolus*.
After World War I (1914-1918) Erni worked as botanist for the administration of South West Africa. After World War II (1939-1945) he resided in Windhoek for some years, during which he developed the botanical garden around the administration building. He then moved to the land of his birth, but soon returned to Namibia and then moved to Tanganyika (now Tanzania), where he worked as prospector and farm manager. One of his sons, Franz Herbert Joseph Erni, maintained a private herbarium at Aus until at least the late nineteen-seventies.