Harry Forbes Witherby was born in Croydon, Surrey on 7 October 1873. After leaving school he joined the family publishing business, Witherby & Co., becoming a senior partner on his father's death. During the First World War he held a commission as Lieutenant, Royal Navy Volunteer Reserve, serving 1917-1918 as a naval intelligence officer at Dunkirk. From a young age he spent as much time as he could studying ornithology. Witherby & Co., as well as publishing books of a general nature, specialised in books on natural history, sport and travel, including ornithological periodicals and standard works edited by Witherby.

Witherby's main ornithological interests were in systematic morphology and plumage changes, geographical distribution and migration. He encouraged field work on British birds and in 1907 founded the periodical British Birds, of which he remained editor-in-chief until his death, for the bringing together of results of such field work. He was also an editor of A Handlist of British Birds (1912), The Practical Handbook of British Birds (1919-1924) and The Handbook of British Birds (1938-1941). In 1933 he played a leading role in the foundation of the British Trust for Ornithology.

He undertook a number of expeditions abroad in order to observe birds and collect specimens. He visited the Delta of Guadalquivir in southwestern Spain (1898), The Kola Peninsula in Russian Lapland with A.E. Hamerton (1899), the White Nile (1900) and southwestern Persia with C.F. Cranburne (1902). In 1904 he married Lillian Gillson who shared his interest in birds and accompanied him on many of his ornithological expeditions, including their honeymoon to Algeria. His later ornithological expeditions included Holland, Norway, Hungary, the Balearic Islands (1919), Portugal (1920) and various parts of Spain on seven occasions from 1921 to 1931.

He was a member of various societies including the British Ornithologists' Union, The British Ornithologists' Club and the Zoological Society of London. He was a Corresponding Fellow of the American Ornithologists' Union in 1916 and an Honorary Fellow in 1928. In 1938 he received the highest award of the British Ornithologists' Union, the Godman-Salvin medal in gold, as a "signal honour for distinguished ornithological work". His committee positions included:

Biographical history written with reference to Witherby's entry in Who Was Who and obituaries in Ibis (vol. 86, no. 2, pp. 208-222), British Birds (1 Jan 1944, p. 142), and The Times (13 Dec 1943, p. 6).