Brian Colquhoun was born on 13 November 1902, the younger son of Arthur Hugh Colquhoun, OBE. He was educated at Felsted School and the University of London (King's College) and in 1936 married Beryl Marquis, youngest daughter of Lieutenant-Colonel H. G. Cowan, MBE; they had two sons.

Colquhoun began his engineering career at Muirhead, Macdonald Wilson and Company Limited, Civil Engineering Contractors. He then joined C. H. Lobban, also becoming a member of the Institution of Civil Engineers (MInstCE). Colquhoun then spent four years in Mexico with the Mexican Eagle Oil Company, before returning home, first as Resident Engineer of the Mersey Tunnel in Liverpool, in 1930, and then as Resident Engineer-in-Charge, from 1933 to 1936. His skills were in demand elsewhere, and he also advised on the Maas tunnel in Rotterdam [the Netherlands], the Tamar tunnel in Plymouth [Devon] and the Rossall sea wall [Lancashire], between 1935 and 1939. On leaving Liverpool, Colquhoun worked as Engineer-in-Chief for the Royal Ordnance Factories at Chorley, Risley and Kirkby from 1936 to 1940, then during 1940 was responsible for the rehabilitation of war production factories in main industrial centres, before serving from 1941 to 1944 as Director-General of Aircraft Production Factories.

In 1944 Colquhoun founded Brian Colquhoun and Partners, Consulting Engineers and Chartered Civil Engineers, working throughout the world, in London, Manchester, Liverpool, Abu Dhabi, Dubai, Saudi Arabia, Sultanate of Oman, Kuwait, Sudan, Ecuador and Paraguay. He was then seconded for two years as Engineering Adviser to the International Bank for Reconstruction and Development, from September 1954, and acted as Engineering Adviser to the Parliamentary Channel Tunnel Committee, from 1956 to 1957.

Colquhoun died on 26 September 1977.