Theodore Heinrich Watermeyer, civil and electrical engineer, was the son of Christiaan Johannes Watermeyer and his wife Carolina Agnita Magrita de Graaff. He was educated at Graaff-Reinet College and the Stellenbosch Gymnasium and from 1893 continued his studies in England, qualifying in civil and electrical engineering at Mason College, Birmingham (which became part of the new University of Birmingham in 1900). After practical training with a consulting engineer specialising in railway work he returned to South Africa and in 1903 was appointed as an assistant engineer in the Cape Government Railways.
He was initially stationed in Port Elizabeth and Humansdorp, working on the Port Elizabeth-Avontuur railway (1905-1906). During 1910-1913 he supervised the difficult construction of the railway from George to Oudtshoorn, through the Montagu Pass, probably the most important engineering achievement of his career. By 1914 he was a resident engineer in the South African Railways and Harbours and around 1916 designed a track-laying machine. From 1918 to 1920 he was assistant superintendent in Pietermaritzburg, but in 1921 was transferred to Johannesburg as assistant chief civil engineer. The next year he represented South Africa at the International Railway Conference in Rome. From 1925 to 1928 he was an assistant general manager, first at Bloemfontein and then in Cape Town, where he supervised the electrification of the suburban railway system. Returning to Johannesburg he acted as general manager from 30 May to 12 September 1931, in the absence of Mr J.R. More. He succeeded More as general manager of the South African Railways and Harbours in February 1933 and held this position until his retirement in 1941. He was an able administrator and during his career was responsible for many expansions and improvements in the South African railway system, including electrification, the expansion of road motor transport, and the establishment of South African Airways.
Watermeyer was a Fellow of the Geological Society of London (FGS) and an associate member of both the (British) Institution of Civil Engineers and the Institution of Electrical Engineers. He was an early member of the Cape Society of Civil Engineers (from 1903) and of its successor, the South African Institution of Civil Engineers (from 1910), serving as its president in 1926.
In 1909 Watermeyer married Jessie Kerr Batchelor, with whom he had a son and two daughters. After her death in 1939 he married Edith Winifred Lomas in 1945.