Theodore MacKenzie, amateur astronomer and historian of South African astronomy, was the secretary of the Cape Astronomical Association when it was revived around 1916, during World War I (1914-1918). The next year he moved to Johannesburg, where he was largely responsible for the formation of the Johannesburg Astronomical Association in February to March 1918 and was elected its first secretary. Unfortunately his contributions to the association were interrupted by illness in December that year. However, in 1921 he was again a member of the association's council. He was probably the Theodore MacKenzie who passed the matriculation examination of the University of the Cape of Good Hope in 1895 and the intermediate examination for the BA degree (equivalent to the first year of study) in 1897.
During 1922-1923 the Cape Astronomical Association and Johannesburg Astronomical Association amalgamated to form the Astronomical Society of South Africa (ASSA), with centres in Cape Town and Johannesburg. MacKenzie was elected as the first secretary of ASSA, for 1922/3. However, he moved to Grahamstown around the middle of 1923 from where he wrote articles on astronomy for the newspaper Die Burger. During 1928/9 ASSA again established a Meteor Section (the previous one having failed owing to a lack of observers) with MacKenzie as its director. In 1929 he was also secretary of ASSA's Computing Section.
In 1920 MacKenzie read a paper before the Cape Astronomical Association on 'Seventeenth century astronomy at the Cape', dealing mainly with the work of G. Tachard*. The paper was published in the South African Quarterly and reprinted as Circular No. 7 of the association. It was followed by several more lectures and published papers or notes on the history of astronomy in southern Africa and other astronomical topics, including the following: 'The story of the southern constellations' (Journal of the Astronomical Society of South Africa, 1925) 'Historic determinations of longitude at the Cape' (South African Journal of Science, 1931), 'Early astronomical observations in South Africa' (Monthly Notes of the Astronomical Society of South Africa (MNASSA), 1941), 'Mason and Dixon at the Cape' (Ibid, 1951), and 'Historic determinations of the longitude of the Cape' (Ibid, 1952).
MacKenzie was a Fellow of the Royal Astronomical Society (FRAS). Collections of his documents, mainly historical but also relating to the total solar eclipse of 1940, were presented to the Cory Library for Historical Research at Rhodes University, Grahamstown.