John Charles Mitchell, son of Charles Mitchell and his wife Janet, was educated at George Watson's College, Edinburgh, and matriculated in 1888. He continued his studies at the University of Edinburgh. After completing two years of medical studies he switched to science and graduated Bachelor of Science (BSc) in 1891 and Doctor of Science (DSc) in 1896. As co-author with J.C. Ewart he published two papers on the "Lateral sense organs of elasmobranchs (Laemargus and skate)" in the Transactions of the Royal Society of Edinburgh (1892, 1895). Meanwhile he had been appointed in 1892 as professor of zoology and botany at the agricultural college near Cairo, Egypt, and also became involved in research for the Egyptian Department of Agriculture. He produced two reports on his work: Report on the edible fishes of Lake Menzaleh (in the Nile delta, 1895) and On the fish fauna of the lower Nile (1896). For this work he was awarded the Order of the Medjidieh by the Egyptian government.
Mitchell returned to Edinburgh in 1898 to continue his studies. He received the Special Certificate in Tropical Diseases and Bacteriology in 1899 and the next year was briefly employed at the Pasteur Institute in Paris. Later in 1900 he qualified as Bachelor of Medicine (MB) and Master of Surgery (CM) at Edinburgh. Meanwhile he came to the Cape of Good Hope, where he entered the civil service in April 1901 as special plague medical officer in Cape Town, during a local plague epidemic. In June that same year he was appointed senior assistant at the Colonial Bacteriological Institute in Grahamstown, directed by Dr Alexander Edington*.
In 1902 Mitchell became a foundation member of the South African Association for the Advancement of Science, but his membership lapsed at some time between 1906 and 1910. On 7 July 1902 he was licensed to practice medicine in the Cape Colony and that same year left the Institute. He became a railway medical officer and in 1906 was practicing at Cookhouse and Somerset East in the Eastern Cape. The University of the Cape of Good Hope admitted him to the DSc degree in 1904, on the basis of the doctorate he had received at Edinburgh, and appointed him as examiner in zoology for its BA and BA (Hons) examinations of 1907-1909. He published "Notes on a peculiar and undescribed affection of the nose" in the Transvaal Medical Journal (1908/9, Vol. 4, pp. 275-276). The paper had earlier appeared in the Journal of Tropical Medicine and contained a description of three cases of ulceration of the nose, perhaps associated with syphilis, that he had observed in the Eastern Cape.
In September 1908 Mitchell succeeded Dr A.W. May* as bacteriologist at the Government Laboratory of the Transvaal Colony in Johannesburg. There he received a grant from the Chamber of Mines to undertake studies relating to pneumonia, which was rife among mine workers. He also served as pathologist to the Johannesburg Hospital. After a short time he resigned his post to become medical superintendent of the recently established miners' phthisis sanatorium at Modderfontein. However, mainly as a result of poor health he soon resigned from this position too. He then opened a private clinical research laboratory in Johannesburg, but died in July 1913. He was survived by his wife Janet and two daughters.