William Reid Dewar studied at the Ontario Agricultural College in the town of Guelph, Canada, obtaining the degree Bachelor of Science in Agriculture (BSA). He came to South Africa in August 1904, aged 22, to take up an appointment as chief of the Biological Division in the Department of Agriculture of the Orange River Colony (now the Free State). He was the first entomologist to be appointed by the government of that territory. His annual report for 1904/5 included a contribution by C. McG. Johnston* on some birds in the territory and their relation to agriculture. Dewar got discouraged with the lack of library and museum facilities at Bloemfontein, and the lack of stimulation for an economic entomologist. Hence, when C.W. Mally* resigned as Eastern Province entomologist, Dewar applied for the position with the concurrence of his department, and the hope that he would be more productive in Grahamstown. He resigned from his post in Bloemfontein on 30 November 1905 to take up his new appointment.
Dewar was a Fellow of the (British) Entomological Society. He became a member of both the South African Philosophical Society and the British Association for the Advancement of Science in 1905, and joined the South African Association for the Advancement of Science at about the same time. He was furthermore a lieutenant in the Grahamstown First City Volunteers. In 1907 he published an article on 'Vine mildew - Plasmopara viticola' in the Agricultural Journal of the Cape of Good Hope (Vol. 31, pp. 324-329). This was followed the next year by a paper on 'The fruit fly: Paraffin remedy versus poisoned bait', which was also published by the Department of Agriculture. At the end of 1907 he resigned his post to return to Canada.