His orders were to bring Christianity to the Nama and the Herero in South West Africa—not an easy task considering that both tribes were enemies at that time, albeit at peace from Christmas Day 1842 to 1846.

When in 1844 Wesleyan missionaries led by Richard Haddy arrived at the invitation of Jonker Afrikaner, Hahn and his colleague Franz Heinrich Kleinschmidt moved northwards into Damaraland in order to avoid conflict with them.

Since Haddy had fled Windhoek in the wake of Jonker's raids, Hahn was tasked to fill the void, but he failed and returned to Germany, arriving in Barmen on 13 September 1853.

His writing about the journey would later be published in the German travelogue Petermanns Geographische Mitteilungen, in which his descriptions of northern Hereroland and the San language, territory, and culture corroborated Francis Galtons reports on Ovamboland.

In 1868, however, an attack by the Nama ended his hitherto successful project, and the Herero under chief Maharero fled the settlement to Okahandja and gave up their Christian affiliations.

When the Rhenish Missionary Society began trading for profit and colonizing (rejecting his Lutheran austerity for a more Reformed Church orientation), Hahn severed his ties with them on 4 March 1872 and returned in 1873 to Germany, by which time 13 missions in Hereroland were prospering.

After failed efforts by Cape Colony to make South West Africa a British protectorate, the Nama-Herero war sparked anew on the "bloody night" of 23 August 1880.

[6] Apart from a treaty between the Swartbooi Nama and the Herero, Hahn was unsuccessful and recommended in his March 1882 report to the Cape government that the Walvis Bay area be maintained as British territory.

As early as 1846, he compiled the first Rhenish Missionary prayer book in Herero, and together with Rath, he released a collection of biblical stories and hymns translated into the language under the title Ornahungi oa embo ra Jehova in 1849.

The latter, including a comprehensive grammar and a Herero-German dictionary of 4,300 words, was the first publication using Standard Alphabet by Karl Richard Lepsius, eventually causing much consternation over its suitability for transcribing a Bantu language.

After his retirement for health reasons in 1884, Hahn visited Margaritha in the United States, and later lived with his son, Carl Jr., in Paarl, then minister of St. Petri's Lutheran Church there.