John Maclure Sinclair qualified as a member of the Royal College of Veterinary Surgeons (MRCVS) in Edinburgh in December 1895. In 1897 he was one of a number of veterinary surgeons appointed in a temporary capacity to help with rinderpest control in the Cape Colony. Arriving in Cape Town on 10 April that year he worked in Kimberley, the Albert District (centred on Burgersdorp), Venterstad, Richmond, and elsewhere, reporting on his activities to the Colonial Veterinary Surgeon, Duncan Hutcheon*.
In 1899 Sinclair joined the Veterinary Department of Southern Rhodesia (now Zimbabwe) and on 1 April 1905 succeeded C.E. Gray* as head of the department. From about 1910 his post as chief veterinary surgeon formed part of the Department of Agriculture and he retained his position until at least 1930. During 1903 he assisted Dr Robert Koch* in Bulawayo in the investigation of East Coast Fever. Later he wrote a brief report, "History of African Coast Fever", which was published as an appendix to the Report of the Committee of Enquiry on African Coast Fever (Salisbury, 1910). An expanded version of this report, entitled "A short history of the infective diseases amongst the domestic animals of Southern Rhodesia since the occupation: African Coast Fever", was later published in the Rhodesia Agricultural Journal (1922, Vol. 19, pp. 10-16). Throughout this period he spent much of his time in the battle against the disease, though various other stock diseases also required his attention. Following an outbreak of rabies near Bulawayo in 1902, he regularly reported on the fluctuating incidence of the disease in his annual reports from 1906 to 1914, after which no more cases occurred in the territory for many years. In 1913 he visited German East Africa (now mainly Tanzania) to obtain information on the possible spread of rinderpest from there. His report was considered at a veterinary conference, held in Bulawayo and attended by representatives of all the southern African states, and recommendations drawn up to limit the possible spread of the disease. He was appointed to represent both Northern Rhodesia (now Zambia) and Southern Rhodesia at the International Veterinary Congress to be held in London in 1914.
Following the appointment of C.K. Brain* as director of agriculture in 1929 the department was reorganised and Sinclair, still holding the post of chief veterinary surgeon, was appointed also as chief of the department's Division of Animal Industry. He became a member of the Rhodesia Scientific Association in 1903. At some time he also joined the Transvaal Veterinary Medical Association (founded in 1903) and remained a member until his membership was transferred to the South African Veterinary Medical Association in 1919.