John H. Tredgold, son of Thomas Tredgold and his wife Elizabeth Harfield, arrived at the Cape of Good Hope from England in 1818 and was licensed to practice as a chemist and druggist on 3 July that same year. [According to Philip (1981) he was the first person to be so registered; however, licensing of apothecaries by the Supreme Medical Committee had already been introduced in 1807]. Around the same time he bought the chemist and druggist business of P.E. Wahlstrand* in Long Street, Cape Town. His mother and stepfather (John Allsopp) lived next door. He married Elizabeth Merrington, with whom he had five children, and was the ancestor of the Tredgold family in South Africa.
Tredgold became a prominent citizen of Cape Town. In March 1822 he was a foundation member of the Commercial Exchange. He was also a foundation member of the South African Literary Society in 1824, but Governor Somerset refused permission for its establishment. Three years later he was a member of the newly established Cape of Good Hope Horticultural Society. In 1833 he served on the committee of the South African Infant School in Cape Town.
For a number of years Tredgold participated in scientific activities at the Cape. In 1825 he was a member of a three man board of enquiry, presided over by Dr James Barry*, to investigate the water supply and the state of the iron water pipes of Cape Town. The board reported on 28 November that year, discussing in some detail the causes of rust in iron water pipes and the results of their experiments to prevent it through the application of various coatings. In 1830, at the request of Dr Andrew Smith*, he analysed a sample of calcareous tufa from Green Point and reported the results in the South African Quarterly Journal (No. 4, p. 445). From about 1833 to 1837 he served on the council of the South African Literary and Scientific Institution. Also in 1833 he was a member of the provisional committee of the Cape of Good Hope Association for Exploring Central Africa, which organised an expedition into the interior led by Dr Andrew Smith*. Later that year he was elected by the shareholders as a member of the association's management committee, and re-elected in March 1836. Tredgold regularly advertised his business in the South African Commercial Advertiser. For example, on 12 March 1834 he informed medical practitioners and the public through an advertisement that he had just received a fresh supply of Indian leeches. In 1836 he appointed John T. Pocock as his assistant and in February the next year they entered into a partnership, practising under the name Tredgold and Pocock. That same month Tredgold returned to England for health reasons, leaving Pocock in charge of the business. The firm was still advertising its wares in the local press in 1839. In England Tredgold was active in the anti-slavery movement, serving as secretary of an important London convention on the subject in 1840. When he died two years later his widow and children returned to the Cape, where her parents had also settled.