Thomas Harvey Johnston was born in Sydney on 9 December 1881. He studied at Sydney University, where he was awarded a BA in 1904 and BSc in 1906. He completed an MA in history in 1907 and his Doctor of Science degree in 1911, specialising in the field of parasitology. Early in 1907 he married Alice Maude Pearce. After lecturing in zoology and physiology at Sydney Technical College for several years he was appointed Assistant Microbiologist in the Bureau of Microbiology in Sydney, where he worked under Dr JB Cleland (see AA 60).

In 1911 Johnston was appointed Lecturer in Biology at the University of Queensland. In the following year he was appointed the Chairman of the Commission of Inquiry to investigate means of controlling the spread of the prickly-pear cactus in Queensland. Between 1912 and 1914 he travelled around the world searching for parasites that might help to contol the cactus pest and played an important role in the introduction of the cochineal insect which ultimately led to the control of prickly pear in Queensland. In 1919 Johnston was appointed the first Professor of Biology at the University of Queensland. He was also appointed Controller of the Commonwealth Prickly Pear Laboratories from 1920-1923. For the significant contribution he had made in controlling prickly-pear, he was awarded the Walter and Eliza Hall Fellowship in Economic Biology.

In 1922 Johnston was appointed to the newly created Chair of Zoology at the University of Adelaide, a position he retained until the time of his death. He maintained his research interests in helminthology (the study of nematodes and other parasitic worms), and became a world authority in that field. He was also interested in marine zoology, and this led to his appointment as Chief Zoologist on two British, Australian and New Zealand Antarctic Research Expeditions between 1929 and 1931.

Soon after moving to Adelaide Professor Johnston became an Honorary Associate of the South Australian Museum, and was a member of the Board of Governors of the Public Library, Museum and Art Gallery of South Australia from 1927 to 1929. Upon Edgar Waite's death in 1928, he was appointed Honorary Director of the Museum, and he remained in that position for three years. He was a member of the Board until 1940, when the three institutions became separate entities with their own boards. Johnston was then appointed Chairman of the Museum Board, a position he retained until his death in 1951.

Johnston and his wife, Alice, had two children, a son and a daughter. Mrs Johnston was interested in Aboriginal welfare and served as a member of the South Australian Advisory Council on Aborigines during the 1930s and a member of the Aborigines Protection Board during the 1940s and 1950s.

During his early years in Adelaide, Johnston developed an interest in Aboriginal anthropology and archaeology. He became a member of the Anthropological Society of South Australia (see AA 824) in 1929, and was elected President in 1938. He participated in Board for Anthropological Research (see AA 346) expeditions to MacDonald Downs (1930), Cockatoo Creek (1931), Mt Liebig (1932), Ernabella (1933), Diamantina (1934), The Granites (1936), Nepabunna (1937) and Ooldea (1939). On these expeditions he assisted JB Cleland in taking blood samples and other medical tests, and also made studies of Aboriginal use of flora and fauna.

Johnston was made a Fellow of the Linnean Society of London in 1911 and a Fellow of the Royal Microscopical Society in 1912. He served as President of the Royal Society of Queensland in 1915 and 1916. After moving to Adelaide he became a member of the Royal Society of South Australia, and served as President from 1931 to 1932. He was also a member of the biology section of the Australasian Association for the Advancement of Science and the Entomological Society of South Australia. Johnston was awarded the David Syme Memorial Medal by the University of Melbourne in 1913 for original scientific research. In 1934 he was awarded the King's Polar Medal for his work in the Antarctic. He was awarded the Sir Joseph Verco Medal by the Royal Society of South Australia in 1935 and the Mueller Memorial Medal by the Australian and New Zealand Association for the Advancement of Science in 1939.

TH Johnston died on 30th August, 1951.