Sir Landsborough Thomson was born in October 1890 in Edinburgh, the eldest child of naturalist John Arthur Thomson. He was educated at Edinburgh's Royal High School, Aberdeen grammar school, and Aberdeen University. He graduated with an MA in 1911, and then went on to study in Heidelberg and Vienna, returning to Britain in 1914, and serving with the Argyll and Sutherland Highlanders in France for the whole of the First World War. After being demobilised in 1919, Thomson was appointed to the Treasury, then transferred to the Medical Research Council in London, where he remained until his retirement in 1957.

Thomson was involved in ornithology from a young age, with his first publication being a note in British Birds when he was seventeen. He undertook the first bird ringing scheme in Britain, which after the war was merged with H.F. Witherby's scheme. He was elected to the British Ornithologists' Union (BOU) at the age of twenty. His publications included Problems of Bird Migration (1926), Birds: an Introduction to Ornithology (1927), and Bird Migration: a Short Account (1936), as well as many papers, an article on migration in the Encyclopaedia Britannica (1929), and, assisted by his wife, he edited and contributed to A New Dictionary of Birds (1964). He held many posts in ornithological and conservation organisations including as chairman of the British Ornithologists' Club (1938–1943) , chairman of the British Trust for Ornithology (1941–1946), president of the BOU (1948–1955), president of the Zoological Society of London (1954–1960), chairman of the Wildfowl Trust's scientific advisory committee (1953–1966), chairman of the Home Office advisory committee on the protection of birds (1954–1969), chairman of the Council for Nature (1964–1969), and chairman of the trustees of the Natural History Museum (1967–1969).

Thomson was awarded the Bernard Tucker medal of the British Trust for Ornithology in 1957, the BOU's Godman-Salvin medal in 1959 and the Royal Society's Buchanan medal in 1962). He was appointed CB in 1933 and knighted in 1953. He died in 1977.