Charles Johnston was the son of a merchant of Manchester, England. He was educated at Guy's Hospital, London, qualifying in 1835 as a member of the Royal College of Surgeons (MRCP) and a licentiate of the Society of Apothecaries (LSA). He led an unsettled life, which included successive periods as a surgeon in the Portuguese army, medical practitioner in London, and surgeon in the English East India Company. In 1839 he jumped ship in East Africa and travelled inland, resulting in his first book, Travels in southern Abyssinia... (1844). This was followed by a second book on Medical etiquette (1847). At this time he was also the editor of the Pictorial Times and the Lady's Newspaper.
Economic depression and the failure of his ventures led him to emigrate to Natal Colony with the Byrne settlers, arriving in October 1849. In December that year he was appointed as the first superintendent of the experimental garden that was being developed by the Natal Agricultural and Horticultural Society. Following the death of his wife, Sarah, in 1850 he resigned his post. He was licensed to practise medicine in the colony in November 1851. Despite a strong urge to explore the interior he remarried, settled in West Street, Durban, and obtained the post of Port Health Officer. He became Durban's best known early doctor and a colourful personality of Natal. From his arrival he was active in local politics and as early as 1850 tried to have a municipality established for Durban. When it was eventually formed in 1854 he served on the town council from 1854 to 1856 and became known for his prominence at public meetings and his argumentative nature. From 1856 he was a member of the newly created Legislative Council, representing Victoria County. He showed an interest in native education and in cotton planting.
Johnston published Observations upon disease in Natal (1857) and Observations on health and disease, and on the physical economy of human life in Natal... (Pietermaritzburg, 1860). The latter was dedicated to the well-known early explorer of Natal, Dr Andrew Smith*, and included a chapter on the geological structure and climatology of the colony. The book was intended for popular use and included much information on the anatomy and physiology of the human body. It was also the first printed work in South Africa to agitate against quackery, though some of Johnston's own ideas were rather peculiar. He returned to England with his eight children in 1860 or 1861, still a poor man.