Reenen Jacob van Reenen, civil engineer, was the son of Albert Johannes van Reenen and his wife Susanna de Villiers. From 1897 he studied at the South African College, Cape Town. He passed the matriculation examination of the University of the Cape of Good Hope in 1899, the Survey Certificate Examination in 1901, and was awarded the degree Bachelor of Arts (BA) with honours in mathematics and natural science (by the University of the Cape of Good Hope) in 1902. A Queen Victoria Scholarship enabled him to go to the United States and qualify as a civil engineer at Lehigh University, after which he worked for a year on large irrigation projects in Nebraska. Upon his return to South Africa he entered the civil service of the Cape Colony in September 1909 as assistant engineer on the Gamtoos River Survey and from 1910 in the Midland Circle Irrigation Department. In May 1912 he was appointed Superintendent of Roads and Local Works in the Provincial Administration of the Orange Free State (now the Free State). Two years later he published a report on The Free State roads (1914; also in Afrikaans).
In later years van Reenen served on government commissions which investigated and reported on a variety of matters including the best means of avoiding losses as a result of drought (1920), matters in connection with irrigation (1924), the financial position of irrigation works funded by the government (1925), and mining of low grade ore on the Witwatersrand. He was a member of the Angola/South West Africa boundary commission (1926) and the irrigation commission (1926, and chairman from 1931), the author of the report of the Drought Investigation Commission on South West Africa (now Namibia) and co-author (with H.S.O. du Toit and others) of the Final Report of the Drought Commission on the Union of South Africa (1923). In 1933 he went on a tour to North and South America, China and Japan to study irrigation projects. He also published a number of papers on topics relating to drought and irrigation, for example, 'A resume of the drought problem in the Union of South Africa' (South African Journal of Science, 1923, Vol. 20, pp. 178-192), 'Development of irrigation in the Union of South Africa' (Ibid, 1925, Vol. 22, pp. 20-41), 'Note on the apparent regularity of the occurrence of wet and dry years in South West Africa' (Ibid, 1925, Vol. 22, pp. 94-95), and Resisting drought (Pretoria, 1935, 221 pp). He was a member of the South African Association for the Advancement of Science and served as president of Section A in 1925.
Van Reenen was intensely interested in prehistoric rock art. He wrote the first book on rock art in Afrikaans, Iets oor die Boesmankultuur (Bloemfontein, 1920, 75 pp), the printed version of a lecture he delivered before Die Suid-Afrikaanse Akademie and based on his documentation of sites in the Free State. He also wrote articles on rock art for magazines such as Die Brandwag (1921) and Die Huisgenoot (1922) and produced a portfolio of 21 of his tracings of rock art from 5 sites in the Free State visited during September-October 1919. This portfolio, titled Boesman tekeninge, is now in the Art Archive of the University of Pretoria (Duffy, 2012).
Van Reenen was interested in painting, music and literature and was a member of the Suid-Afrikaanse Akademie vir Taal, Lettere en Kuns. He wrote and illustrated several early fictional works in Afrikaans during 1917-1921 and was the first significant author of short stories in Afrikaans. In December 1908 he married Elizabeth Lilian Roos, with whom he had a son and two daughters.