J. Theophilus Hahn, trader and Nama linguist, was a son of the Rhenish missionary Johannes Samuel Hahn and his wife Helene Langenbeck, and elder brother of Paul D. Hahn*. He grew up at the mission stations Ebenhaeser and Bethanien in southern Namibia, with Nama as his second language. At the age of seven he was sent to Barmen, Germany, to start his schooling and his parents joined him there three years later. He was trained as a surveyor and cartographer, but then changed direction and for four years studied philology at the University of Halle, qualifying as Doctor of Philosophy (PhD) in 1870 with a thesis titled Die Sprache der Nama. The thesis was published that same year with an appendix on the myths of the Khoi. He appears to have been a good scholar at this time. In addition to his knowledge of German, Nama and Herero he had a knowledge of English, French, Dutch, Latin and Greek; and while still a student published several papers (in German) in the journal Globus, including a fine and very early ethnographic study of the Nama (1867) and a valuable early description of the Bushmen (1870). He also wrote "Beiträge zur Kunde der Hottentotten" (Contribution to the knowledge of the Hottentots), an extensive paper published in the Jahresbericht des Vereins für Erdkunde, Dresden (1868-1869).
Hahn stayed in the Cape Colony for a brief period, during which he married Marian Smuts at Stellenbosch on 8 May 1875. They had one son. He then settled among the Nama at Swartmodder (now Keetmanshoop) as a trader. However, owing to the unsettled conditions in the region he was not successful and moved to Rehoboth. Here his quick temper and aggressive nature caused him to clash with the Baster Council, while he also antagonised Maharero, Chief of the Hereros, by advocating that Transvaal Boers should settle at Rehoboth and in northern Namaland. As a result various chiefs asked the Cape government for protection and W.C. Palgrave* was sent to the territory in 1876 to investigate. Hahn left Namibia in 1878 and settled at Stellenbosch. Here he worked on the first topographic map of Namibia, Original map of Namaqualand and Damaraland compiled from his own observations and surveys by Th. Hahn, PhD (scale 1:742 016). Points are indicated on the map where latitudes were determined from sun or star culminations and longitutes from lunar observations. The map was drawn in the Surveyor-General's office in Cape Town and issued in October 1879. Other publications by him at this time were a paper on "The graves of Heitsi-eibib: a chapter on the prehistoric Hottentot race" in the Cape Monthly Magazine (1878, Series 2, Vol. 16, pp. 257-265); a brief report (in German) on rock paintings in the Zeitschrift für Ethnologie (1879); and a book, Tsuni-//Goam: the supreme being of the Khoi-Khoi, published in London in 1881, which is however not highly regarded. None the less his work contributed significantly to knowledge of the life, culture and religion of the Khoi and San. He became a corresponding member of the Geographische Gesellschaft, Dresden; the Gesellschaft für Erdkunde, Berlin; and the Anthropologische Gesellschaft, Vienna.
In February 1881 Hahn was appointed Government Philologist and warden of the Grey collection in the South African Library in Cape Town. He immediately became a member of the South African Philosophical Society upon moving to Cape Town, remaining a member for some seven years. In April 1882 he delivered a lecture at the South African Library, "On the science of language and its study, with special regard to South Africa", which was published as a pamphlet. That same year he read a paper before the South African Philosophical Society on "Early African exploration up to the end of the 16th century", which was published in revised form in the Cape Quarterly Review (1882, Vol. 1(4), pp. 689-713). Two years later he completed An index to the Grey collection in the South African Public Library, which remained useful for a century despite its bibliographical defects and inaccuracies. However, by this time he had already resigned from his post, following criticism of his unsatisfactory work as a librarian, and returned to Stellenbosch in November 1883.
In 1889, after some difficult years, Hahn again went to Namibia, this time as an agent for the Kharaskhoma Exploring and Prospecting Syndicate, for which he obtained land and mineral rights. Upon his return to Stellenbosch he bought a farm, but by 1898 he was insolvent. After the Anglo-Boer War (1899-1902) he was employed by a mining company in Johannesburg, where he died of cancer. He was quick-tempered and arrogant, and these traits contributed much to the problems he experienced throughout his life.