Frederick Gordon Cawston was the son of Samual Cawston and his wife Agnes Isabella Boys. He studied at the University of Cambridge, where he passed the honours examination in natural history for the Bachelor of Arts (BA) degree in 1906. Continuing his studies he qualified as a member of the Royal College of Surgeons of England (MRCS) and a licentiate of the Royal College of Physicians of London (LRCP) in 1909 through St Thomas's Hospital in London. The next year he was awarded the degree Bachelor of Surgery (BCh) by the University of Cambridge and in 1917 qualified there as Doctor of Medicine (MD). He was elected a Fellow of the Royal Society of Arts.
After some time in 1909 as house surgeon at the Seaman's Hospital at the Royal Albert Docks in London Cawston came to Natal in 1910 to take up an appointment as medical officer at Grey's Hospital in Pietermaritzburg. He was licensed to practise medicine in South Africa in August 1911. By 1915 he was stationed in Greytown, Natal, but in 1917 became medical officer of health in Krugersdorp, Transvaal. The next year, towards the end of World War I (1914-1918) he was at the military hospital in Potchefstroom, with the rank of captain. In 1919 he was in Durban, but from that year to 1922 returned to Britain as First Streatfield Research Scholar. He returned to Durban, where he was still active in 1926 and where he appears to have remained for the rest of his life. He was married to Annie Constance Waters.
One of Cawston's research interests was schistosomiasis (bilharzia). In 1915 he and the zoologist Ernest Warren* suggested that the water snail Physopsis africana was the intermediate host of Schistosoma haematobium, the parasite causing bilharzia. He published three papers on the disease in South Africa in the South African Medical Record in 1916 and warned of the dangers of river bathing in an unpublished paper read at the annual congress of the South African Association for the Advancement of Science that same year. Subsequent papers by him on the disease included "Antimony and emetine in bilharzia disease" (The Lancet, 1921), "Wild birds as a cause of the spread of bilharzia infection" (Journal of Tropical Medicine and Hygiene, 1921), "Schistosomiasis at the source of rivers" (South African Journal of Science, 1932), and "Some past and present aspects of the bilharzia problem" (South African Medical Journal, 1948). His interest in indigenous medicine is shown by a paper on "Native medicines in Natal" in the South African Medical Journal (1933).
However, his main research focus was on the organisms that carry or transmit bilharzia and other parasitic diseases. A number of the resulting papers were published in the Transactions of the Royal Society of South Africa, for example, "Experimental infestation of fresh-water snails" (1921), "Trematodes [a group of parasitic flatworms] and their intermediary hosts" (1924), "The smaller South African shells that harbour cercariae [the final larval stage of trematodes]" (1925), and "Climatological changes and their effect on fresh-water molluscs" (1934). Other papers appeared in the South African Journal of Science, including "Some South African snails and the cercariae which attack them" (1918) "South African cercariae" (1919), "Occasional hosts of some South African trematodes" (1923), and "A consideration of the antimony content in drugs used for the destruction of schistosomes" (1935). A further two papers on larval trematodes from snails appeared in the South African Journal of Natural History (1920, 1921). He also published many papers on the same or related topics in journals such as the Journal of Tropical Medicine and Hygiene, Parasitology, and the British Medical Journal. As a result of his work on fresh-water snails two species, Ferrissia cawstoni and Tomichia cawstoni, were named after him.
Later in his career Cawston developed an interest in animal dentition. Among others he studied the "Succession of teeth in sharks" (British Dental Journal, 1938) and wrote a book on The evolution theory in its relation to tooth replacement (Cape Town, 1948). Outside medicine and biology his interests included rock art, on which he published two papers in the South African Journal of Science: "A consideration of the Bushmen's paintings at Quthing" (1931) and "A selective study of South African Bushman paintings" (1932).
Cawston was a member of the Royal Society of South Africa and became a member of the South African Association for the Advancement of Science in 1916. He was a member also of the South African Biological Society. In addition to scientific research, his leisure activities included conchology and music. Some of his non-scientific activities are reflected in his publications Talks to men in gaol (Pretoria, 1930) and Lessons from the life of Jesus (London, 1930).