Jan (or Johannes) Hartog (or Hartogh, Hertogh, De Hertogh, Hartogius), whose parents settled in Holland after his birth, was employed in the garden of the University of Leiden. The garden was then under the curatorship of Hieronymus Meijer, who was succeeded by Hartog's brother, Willem de Hertogh. While employed as gardener Hartog probably attended some classes in botany. He was later employed by the Dutch East India Company and in December 1690 was sent to the Cape of Good Hope as a naval cadet in the Pampas, travelling with H.B. Oldenland*. In a letter from the directors of the Company to Governor Simon van der Stel*, recommending him for employment, he is described as having a knowledge of local and foreign plants, particularly their names and cultivation, and of whatever else may be required from a herbalist. Arriving in May 1691 he was put in charge of the Company's garden in Cape Town until the appointment of Oldenland as master gardener in 1692 or early in 1693. For the next five years he appears to have retained some role in the garden, and after Oldenland's death succeeded him as master gardener in 1698. His main responsibility was to produce vegetables for the provision of passing ships; however, he also added many indigenous plants to the garden.
Hartog was a member of several cattle trading expeditions into the interior, his brief from Governor W.A. van der Stel* (who succeeded his father in 1699) being to collect indigenous plants and seeds and to assess the agricultural potential of the regions visited. From November 1699 to February 1700 he accompanied an expedition under Ensign Olof Bergh to the Riviersonderend, sending back a consignment including four kinds of aloes, two kinds of bulbs, sixty-two kinds of seeds, and a collection of dried plants. In 1705 he went on an expedition northwards to the upper Olifants River led by Johannes Starrenburg*, landdrost of Stellenbosch. On that journey he collected bulbs also near the mouth of the Langvlei River, near present Leipoldtville. It has been suggested (Mossop, 1927) that at this time he probably traded for cattle independently on the governor's behalf. And in 1707 he accompanied a cattle trading expediton eastwards to the junction of the Riviersonderend and Breede River, south towards the Bredasdorp Mountains, and to the hot springs near present Caledon.
Both Governor Simon van der Stel and his son W.A. van der Stel sent plants, bulbs and seeds to Holland, most of which must have been collected by Oldenland and Hartog. Much herbarium material from these two collectors went to Professor Paul Hermann* at Leiden. Other specimens were described in C. Commelin's Horti medici amstelodamensis rariorum... in 1697 and 1701. Another consignment ended up in the herbarium of Francois Kiggelaar, curator of the garden of Simon van Beaumont at Leiden. A substantial portion of Kiggelaar's collection consisted of Cape plants that were sent to him by W.A. van der Stel in 1700. Kiggelaar's herbarium was later incorporated into the herbarium of Sir Hans Sloane in London, which in turn ended up in the British Museum (Natural History). In 1720 H. Boerhaave published descriptions and illustrations of 24 species of Cape proteas in his Index alter plantarum..., said to be based on drawings and descriptions sent by Hartog. However, the quality of the drawings suggest that they were made by Oldenland. A list of the Cape plants collected by Hartog and Oldenland was included in J. Burman's Thesaurus Zeylanicus in 1737. The famous Swedish botanist Carl von Linne (Linnaeus), in his Flora Capensis (1759), named Hartog as one of the first three leading botanists to collect at the Cape of Good Hope, the other two being Hermann and Oldenland. The genus Hartogia (family Celastraceae) was named in his honour by C. von Linne the younger, the type species being Hartogia capensis, a small tree commonly known as the Ladle wood. He is also commemorated in the name of the proteaceous shrub Mimetes hartogii.
During the events that led to the dismissal of Governor W.A. van der Stel in 1706 it was alleged that the governor had employed Hartog to lay out the grounds on his farm, Vergelegen, while he was being paid by the Company. It seems that Hartog was then transferred to Ceylon, from where he sent a collection of plants and seeds to Cornelius Voss in Amsterdam. Voss passed them on to Burman, who used them in the preparation of his Thesaurus Zeylanicus. Hartog eventually returned to Holland in 1715. The next year he was sent to Surinam, where he died in 1722.