Hendrik Prehn (who called himself von Prehn) was the son of Henning J. Prehn and his second wife Hendrina Schenk. He was sent to Holland to be educated in 1752, where he joined the army of the States-General and achieved the rank of lieutenant. In 1755 he joined the Hanoverian army and by 1760 had attained the rank of captain in the Braunschweig Hussars. After being wounded in the Seven Years War he rejoined the army of the States-General and in July 1768 returned to the Cape with the rank of major. There he was appointed commandant of the troops and was in charge of the garrison, the burgher forces, and the arsenal. As commandant he also became a member of the Council of Policy. In 1770 he married Catharina M. Kirsten, with whom he had three children, including a son also named Hendrik.
Prehn spoke Dutch, German and English, and was regarded as a well-informed and accomplished man, with an interest in science. When Captain James Cook touched at the Cape on his second expedition in 1772, Prehn introduced two members of the expedition, the zoologists J.R. Forster* and his son J.G.A. Forster*, to the visiting Swedish naturalist Anders Sparrman*. This resulted in Sparrman joining the expedition.
Prehn discovered a new mineral, a silicate of calcium and aluminium, in a Jurassic-age dolerite in the Cradock district of the Cape Colony. It was analysed and named prehnite after the discoverer in 1788, by the famous German neptunist geologist A.G. Werner. The mineral was the first to be named and described from South Africa, and was also the first to be named after a person.
In 1779 dissatisfied free burghers submitted a petition to the directors of the Dutch East India Company containing charges against several senior officials, including some against Prehn. The charges against him were not taken seriously and he was honourably discharged from his post early in 1780. He assisted William Paterson*, who was in financial difficulties, and both Paterson and the Prehn family left for Europe on De Held Woltemade in March 1780. His successor at the Cape was Colonel R.J. Gordon*. Prehn took a collection of dried plants with him to Europe. Descriptions of 22 species, with four coloured plates illustrating Pelargonium, were published by A.W. Roth in a short paper, "Observationes plantarum e Capite Bonae Spei", in Botanische Abhandlungen und Beobachtungen (Nuremberg, 1787, pp. 53-65). None of the species was new.
Prehn died at Heilbronn, Germany, while travelling. His widow settled there for the rest of her life.