Thomas Stewart, a civil engineer who specialised in the construction of water works, was employed as an engineering assistant in the Glasgow Waterworks and studied engineering at the Glasgow College of Science and Arts. He became a member of the (British) Institution of Civil Engineers and of the Mineralogical Society, and was elected a Fellow of the Geological Society of London. After working briefly for a consulting engineer in Scotland he came to South Africa in December 1882, having been appointed assistant to the hydraulic engineer of the Cape Colony, John G. Gamble*, on a three year contract. In this position he was concerned mainly with irrigation schemes and the water supplies of towns. Occasionally he acted as an examiner for the University of the Cape of Good Hope, setting a paper on geology, mineralogy and crystallography for BA students in 1884, and one on geology for mining students in 1896.
In 1885 Stewart contributed a paper on "Underground water supply with special reference to the Colony" to a book edited by C. Cohen, The South African Exhibition, Port Elizabeth, 1885 (pp. 225-254). Noting the presence of underground water at shallow depths all over the Karoo, he pleaded for an extension of the government's investigation of ground water resources to greater depths, with a view to meeting future water needs. In 1886 a paper by him on "The rainfall of the Cape Colony" was published in Nature. That same year he left government service to practice as a consulting engineer. For many years his office was situated in St George's Chambers, Cape Town.
Stewart was closely associated with various shemes to augment the city and suburban water supplies. For a number of years he worked on the design and construction of water reservoirs on Table Mountain and during this period lived on the mountain and did mountain climbing in his spare time. The design and management of the reservoirs required observations to determine the distribution of rainfall over the mountainous terrain, an enterprise in which he became intimately involved. From the beginning of 1896 to the end of 1903 he was in charge of a second order meteorological station at Disa Head, including an evaporation station from 1899. His observations were supplied to the Meteorological Commission of the Cape Colony and were summarised in the commission's annual reports. Many years later he published a paper on "The rainfall on Table Mountain" in the Transactions of the Royal Society of South Africa (1913, Vol. 8, pp. 41-46) in which he reported monthly rainfall totals for eleven rainfall stations situated at various elevations around the mountain. The results demonstrated the great increase in rainfall from the base to the top of the mountain.
In January 1906 Stewart submitted a Report on the various schemes which have recently been suggested for an additional supply of water to the Cape Peninsula... (8p) to the Suburban Joint Water and Drainage Committee. By 1915 he was a member of the city's Board of Engineers, appointed by the Waterworks Committee of the municipality, the other two members being W.A. Tait (chairman) and D.E. Lloyd Davies. The board that year reported on the augmentation of the city's water supply, with a supplementary report following in December the next year. In 1923 the board, then consisting of Stewart, Lloyd Davies and F.E. Kanthack*, reported on the Steenbras scheme as an additional water supply for the city.
Stewart was involved in the design of water works also in other parts of South Africa, including the Zuurbekom waterworks for Johannesburg, the water supplies of Worcester, Oudtshoorn, Mossel Bay and other South African towns, and an addition to the water supply of Beira, Mozambique. During the Anglo-Boer War (1899-1902) he served in the Royal Engineers with the rank of major and was concerned with the design and construction of defence works. He married Mary M. Young in 1902 and they eventually had three sons.
In 1895 Stewart was appointed as a member of the newly established Geological Commission of the Cape of Good Hope, which oversaw the systematic geological mapping of the colony by G.S. Corstorphine*, A.W. Rogers*, E.H.L. Schwarz* and others. Stewart, Corstorphine and H. Saunders*, constituting a sub-committee on deep artesian well boring, reported on their work in the commission's annual report for 1896. Stewart served on the commission until it was replaced by the Geological Survey after the formation of the Union of South Africa in 1910.
In 1883, shortly after his arrival at the Cape, he became a member of the South African Philosophical Society, serving on its council from 1893 and as president during 1897-1899. He remained a member when it became the Royal Society of South Africa in 1908 and as late as 1924 published a paper on "Holtzhuisbaaken spring, Cradock" in its Transactions (Vol. 11, pp. 171-190). In 1903 he joined the Cape Society of Civil Engineers soon after its formation, serving on its first committee (1903), as its second president (1904), and as joint vice-president from 1905 until at least 1907. At a meeting of the society in February 1905 he read two papers, one on "Irrigation in Valencia", the other, "Notes on the Puentes Dam", dealing with a 50 m high masonry dam in Spain that failed in 1802 with the loss of over 600 lives. Neither paper appears to have been published. He became a member of the British Association for the Advancement of Science in 1901.
Stewart lived in Wynberg for many years. From 1924 to 1927 he was chairman of the local Society for the Prevention of Cruelty to Animals (SPCA). Following the death of his wife in 1921 he married Georgina Rees, widow of F.R. Thompson, in 1928.