Johannes (Jan) W. Wernich, surveyor, was the son of Joachim J.L. Wernich and his second wife, Johanna S. Beck. He went to the Netherlands, presumably for his education, and in 1783 was appointed by the Dutch East India Company as their surveyor at the Cape for a contract period of five years. After returning to the Cape in 1784 he married Alida W. Munnik on 13 November 1785, but they had no children. He was promoted to the rank of junior merchant in 1794. Following the annexation of the Cape by the British in 1795 he was appointed as surveyor by the new administration on 15 September that year.
Wernich became one of the group of able surveyors and cartographers brought together by Governor C.J. van de Graaff* from 1785. The group included the governor's son, Captain S.W. van de Graaff*, L.M. Thibault*, D.M. Barbier*, J.C. Friderici*, C.H. Leiste*, and F.R. Duminy*. Wernich worked in various parts of the colony and among others compiled a sketch map of the district of Vier-en-twintig-riviere (1787); a map of the main route from Cape Town, via Hottentots Holland, the Overberg and the warm baths at present Caledon, to Swellendam and Graaff-Reinet (1788, revised in 1795); a plan of the Berg River, from its mouth in St Helena Bay to Vier-en-twintig-riviere, with an extensive legend (not dated); a plan of the Dutch East India Company's estate at Newlands (1791); and a map of part of Outeniqualand between Mossel Bay and Plettenberg Bay (not dated). In 1804, when the Cape had been handed over to the Batavian Republic, Governor Janssens sent Wernich eastwards as far as Plettenberg Bay. He kept a diary of the journey and compiled maps of Fisch Bay, Vlees Bay, and Mossel Bay. The next year he was sent to Saldanha Bay and St Helena Bay, again keeping a diary, and produced a good map of Saldanha Bay and the farms around it.
After the Cape again became a British colony in 1806 Wernich became a sworn land surveyor in private practice. He gave up surveying at the beginning of 1811 as a result of poor health and bought the farm Olifantsfontein near Malmesbury. Over the years he had also acquired several houses in Cape Town. His health improved during 1812 and he was re-admitted as a sworn land surveyor on 30 October that year. He is listed as a sworn surveyor in the directory of the African Court Calendar during 1810-1811 and again during 1813-1814, at several different addresses in Cape Town over this period. He became ill again during 1813 and died early in 1814. His maps are preserved in the National Archives, Cape Town, while 168 property maps drawn by him between 1784 and 1800 are kept in the office of the Registrar of Deeds (Cape of Good Hope, 1859).