Johann Theophilus Hahn, son of the Rhenish missionary J. Samuel Hahn, was born at the mission station Ebenaeser (Lutzville) on 24 December 1842. As a boy at the mission stations Ebenaeser and Berseba and Bethanien in South West Africa (Namibia) he learned to speak the different Khoisan dialects. Samuel Hahn left for Germany in 1852, for the sake of his children's academic training. There Theophilus proved himself as an outstanding student and he obtained his doctorate in 1870 on the dissertation Die Sprache der Nama;
nebst einem Anhang enthaltend Sprachproben aus dem Munde des Volkes. While studying at the University of Halle he published a number of ethnological studies relating to the aborigines of South West Africa (Namibia).
On his return to South Africa, he became a trader in South West Africa. After his marriage in 1875 to Marianne Esther de La Roche Smuts he settled at Rehoboth, where he started a trading business. Hahn left South West Africa in 1878, after it became clear that the Cape Government's enthusiasm for action north of the Orange River waned, as a result of its own military burdens on the eastern border, in Griqualand West and Basutoland. He went to live with his brother Johannes, a Rhenish missionary at Stellenbosch, where he
completed the first map of South West Africa.
In 1881 he was appointed as government philologist and custodian of the Grey Collection in the South African Public Library. His appointment as Grey custodian gave rise to an application to the Supreme Court to have the care and custody of the Grey Collection given up to the Grey Trustees and for an interdict to restrain Hahn from interfering with the Grey Collection. This application by the Grey Trustees was refused by the Supreme Court. In 1883 he provided the Cape Native Laws and Customs Commission information regarding the Nama and Herero cultures. This commission was directed by the Cape Government to enquire into the laws and customs of the Blacks and to report on the advisability of introducing some system of local self-government in the Black territories. While living at his farm Prospect Hill, he wrote a letter to the agent of Adolf Luderitz,Heinrich Vogelsang, in which he supplied valuable information about the prospects of South West Africa and suggested that Luderitzacquire Angra Pequena (LuderitzBay) as a harbour for trading purposes. He resigned as philologist and Grey Librarian in 1883 and became the manager of the farm Welmoed, in the district of Stellenbosch. As a wine-farmer he proposed the establishing of a co-operative for the production of wine, also prompt action for combating the phylloxera disease. With the assistance of Hahn the mining company Kharaskhoma Exploring and Prospecting Syndicate, a company based in London, obtained in 1890 important concessions from the chiefs of Bondelswart and Veldskoendraers - a monopoly for the exploitation of minerals, rights to trade and to construct railways. After the syndicate transferred its rights to the South African Territories Company, he left South West Africa and bought the farm Blaauwklip (Blaauwklippen) at Stellenbosch. His farming attempts proved a disaster and his farm was sold to Cecil John Rhodes. He became an agent for the Equitable Life Assurance Society in Johannesburg and while residing at 206 Market Street he offered to act as a spy for the British Military Government. This was declined.
He died on 22 January 1905 and was buried in the Braamfontein cemetery.