Otto Friedrich Mentzel (also Mentsell, Menzel, Mensel) was the son of the Prussian privy councillor and court physician Johann C. Mentzel and his wife Loysa M. Hendreich. Otto was educated at the Joachimsthal Gymnasium, Berlin, and then appointed to a junior post in the treasury in Berlin. After some years he proceeded to Amsterdam to enlist in the service of the Dutch East India Company and was sent to the Cape of Good Hope as a soldier, arriving on 5 July 1733. Though not trained as a teacher he was seconded as such to a wealthy farmer, Paul Keijser, from October 1734 to February 1736, being sufficiently educated to teach young children. From 1737 to 1740 his official position was that of a clerk in the timber store at the Castle, a post he obtained through his patron and fellow Prussian, Lieutenant Rudolph S. Allemann. Meanwhile he stayed for free with Allemann's family for four years, in exchange for which he taught their three children. During 1737 he furthermore taught the young son of the retired Governor Lafontaine for some time. His undemanding position left him time also to earn additional income by designing patterns for embroidering articles of female clothing, and wedding ornaments. He was fluent in Dutch and hence in a position to collect information about the Cape and its inhabitants.
Early in January 1741, while delivering some letters to the ship Hartenlust, bad weather prevented him from being taken off. The ship sailed for Holland that night, with the result that he departed from the Cape, unexpectedly and inadvertently, without any of his possessions. He settled in Silesia (which had just been ceded to Prussia by Austria) and entered the Prussian civil service. He appears to have remained there for the rest of his life, becoming chief of police before his retirement.
Mentzel retained his interest in the Cape, reading the works of Peter Kolb(e)*, Abbe N.L. de la Caille*, Anders Sparrman*, and C.P. Thunberg*. Eventually he read the Nieuwste en beknopte beschrijving van de Kaap de Goede Hoop, by Allemann and Klockner, in the form of a German translation published in 1779. This inspired him to write the Lebens-Geschichte Herrn Rudolph Siegfried Allemanns, gewesenen Kapitaens der Miliz, Oberhaupts der Garnison und Kommandant der Festung... (Glogau [now Glogow, Poland], 1784). An English translation, Life at the Cape in the mid-eighteenth century. Being the biography of Rudolph Siegfried Allemann.., was published by the Van Riebeeck Society in 1919. In addition to the life of Allemann it describes some of Mentzel's activities and his unexpected departure from the Cape (pp. 136-145).
However, Mentzel's major work was his Volstaendige und zuverlaessige geographische und topographische Beschreibung des beruehmten und in aller Betrachtung merkwuerdigen afrikanischen Vorgebirges der Guten Hofnung..., published in two volumes at Glogau in 1785 and 1787. The work was translated into English and published by the Van Riebeeck Society as A geographical and topographical description of the Cape of Good Hope (3 vols, 1921, 1925, 1944). It deals with the geographic features (boundaries, bays, rivers), climate, governance, military organisation, finances, trade, social life, agriculture, viticulture and animal husbandry of the Cape, the lives and customs of both the settlers and the indigenous population, expeditions undertaken by the colonists, and descriptions of Cape Town and the rural districts. The first volume of the German edition is the more authoritative. Mentzel was a born narrator and despite some errors of fact his books provide a comprehensive description of the colony. Much of the material was based on personal observation, but his accounts of events that occurred after his departure are based on the writings of others. One of his insightful comments was that the Cape, with its fewer than 20 000 settlers at the time of his stay, could easily support a million Europeans. He expressed regret at the lack of tree-planting and warned against the waste of natural resources. His books had little influence in his own time and became quite rare.