Thomas Quentrall joined the civil service of the Cape Colony in July 1889 as a mining engineer at Kimberley. In July 1891 he was appointed inspector of mines, Kimberley, and in August 1902 inspector of mines for Kimberley and Barkley West - a position he held to 1910. He was a member of the (British) Institution of Mechanical Engineers (MIMechE) and a Fellow of the Geological Society of London (FGS). From 1898 to 1903 he was an examiner in mining for the second mining examination of the University of the Cape of Good Hope. In 1902 he became a foundation member of the South African Association for the Advancement of Science, serving as president of its Section B (which included geology) at the association's meeting in Kimberley in 1906. He was still active in the association at Kimberley in 1907, but by 1911 was no longer a member.
In 1905 Quentrall and J.H. Ronaldson* were appointed as commissioners to inquire into the mineral resources of Namaqualand with a view to consideration by the Cape government of the advisability of taking over the Port Nolloth to O'Kiep railway. Their report was issued as Parliamentary Report G11-05 (Cape Town, 1905, 20p). Quentrall served on the Board of Management of the McGregor Museum, Kimberley, in 1908 and again in 1911, but left Kimberley in the latter year. He had been an important collector of specimens for the museum since its inception in 1907. The year after his departure he presented the museum with a set of copper ores from Namaqualand.