Theodore Reunert, son of Jerome Reunert, professor of modern languages and his wife Louisa Heald, attended private schools in Leeds, England, then the Realgymnasium (High School) in Cassel, Germany, and studied engineering at the Yorkshire College of Science (now the University of Leeds). After serving an apprenticeship in mechanical engineering with the firm Kitson & Co. of Leeds he became an assistant at a leading electrical firm, the Volt Works in Leeds.
Reunert came to South Africa in 1879 and worked as an engineer at the Orion Diamond Mining Company in Kimberley and later as a consulting engineer in partnership with Edward Jones. About 1884 he established his own firm, "T. Reunert, Engineer and Machinist" (later T. Reunert and Company), which thrived in Kimberley for many decades. He was elected a member of the (British) Institution of Mechanical Engineers in 1883, and a member of the Institution of Civil Engineers in 1889, serving on the South African advisory committees of both bodies. In 1888 he moved to Johannesburg (where gold had been discovered two years earlier) and, in partnership with Otto Lenz, founded the engineering firm Reunert & Lenz in 1889. He remained on the Witwatersrand for the rest of his career. His firm, which was always closely associated with local manufacture, developed into one of the largest of its kind in South Africa. In 1885 he married Catherine E. Green, with whom he had four sons.
Reunert's knowledge of the diamond mines around Kimberley led to several publications. In 1886 he wrote "Diamond mining at the Cape" for inclusion in the Official handbook... of the Cape of Good Hope (Cape Town, 1886, pp. 178-221). It was one of the earliest substantial articles on the mines at Kimberley, Bultfontein and Dutoitspan, and included a description of the geology of the region. An expanded and updated version was published as "The diamond mines" in the Illustrated official handbook of the Cape and South Africa (1893, pp. 323-356). This article, or an extended version of it, was initially written for the South African and International Exhibition held in Kimberley in 1892 and was published as a monograph entitled Reunert's diamond mines of South Africa (London, 1892). It was the first work of note to deal exclusively with the diamond mines around Kimberley and in the Free State. A more comprehensive treatise by him was his book Diamonds and gold in South Africa (Cape Town, 1893, 242p).
As a leading figure on the Witwatersrand for many years Reunert played a significant role in the origin and development of several scientific societies.He was a member of the short-lived South African Geological Society, founded in Grahamstown in 1888. In 1892 he was a foundation member of the South African Association of Engineers and Architects, served on its first council, was president in 1897-1898, and was later elected an honorary life member. In a paper delivered to members in 1896 he discussed "A rational sewerage system". He was a foundation member also of the Geological Society of South Africa in 1895 and served on its first council, but remained a member for a few years only. In 1901 he called a meeting in Cape Town to consider the formation of an annual congress of engineers. This initiative led to the formation of the South African Association for the Advancement of Science in 1902. Reunert and J.D.F. Gilchrist* served as the first joint honorary secretaries, Reunert being responsible for the Transvaal, Natal and Orange Free State from the Johannesburg office. In his role as secretary he petitioned the government of the Transvaal Colony in July 1902 on the need to found a meteorological and astronomical observatory near Johannesburg. His proposal led to the establishment of the Transvaal Observatory, directed by R.T.A. Innes*. Possible sites for the observatory were inspected by Reunert and the surveyor E.H.V. Melvill* on behalf of the association in March 1903. Reunert served as president in 1905, when the association met jointly (in South Africa) with the British Association for the Advancement of Science. Meanwhile he became a member of the South African Philosophical Society in 1901 and when it developed into the Royal Society of South Africa in 1908 was elected one of its first Fellows. In 1918 he was elected joint vice-president of the newly established Johannesburg Astronomical Associan.
Reunert played a leading role in the provision of university, technical, and secondary education in the Transvaal. In November 1894 he held a discussion with the council of the South African College, Cape Town, on their plans to establish the South African School of Mines, and in 1897 helped with arrangements to receive its first students in Johannesburg for practical training. In 1895 he was one of the founders of the Witwatersrand Council of Education and became its chairman in 1906. The purpose of the council was to provide education for English speaking pupils. In 1903 he served on the Technical Education Commission appointed by the government of the Transvaal Colony to make recommendations with regard to technical and university education. Its recommendations led to the establishment of the Transvaal Technical Institute, with Reunert serving as a member of its first council. During subsequent years Reunert was one of a handful of persons who dominated the university movement in Johannesburg, until the institute developed into the University of the Witwatersrand in 1922. Reunert served on the council of the university, which conferred its first honorary Doctor of Literature (DLit) degree on him for his services to education and culture. In 1905 he was a member of the Secondary Education Commission appointed by the government of the Transvaal Colony. The commission's work resulted in the foundation of three high schools in Johannesburg. He also served on the Government Council of Education during its entire existence from 1907 to 1915.
Reunert's main contrubion to cultural development on the Witwatersrand was as one of the founders of the Johannesburg Public Library. He served as a member its committee from before the library was opened until 1913, with only a short break, and acted as chairman from 1897 to 1902. His view of the library as a cultural centre for the community included support for the provision of lectures, museum exhibits, and other activities. With the cooperation of the library and the South African Association for the Advancement of Science he participated in the formation of the South African Lectures Committee, which arranged lecture tours in South Africa for several distinguished overseas lecturers between 1907 and 1910. In 1911 he published Thoughts on education from Matthew Arnold, and in 1915 translated 1815-1915, by Ch. Seignobos, for the Witwatersrand Council of Education.