[2] Harry was the second surviving son of Henry Forbes Witherby (1836–1907) of Holmehurst, Burley, Hants, the owner of the legal and maritime stationer Witherby and Co., employing 169 men who retired from the family business in 1899 to focus on his interests in painting and ornithology, leaving it to be run by his sons.

The family firm of H F and G Witherby, originally printers, began to publish bird books early in the 20th century.

From an early age Witherby devoted himself to the study of ornithology, travelling extensively, including visits to Iran, the Kola Peninsula, and the White Nile.

[8] He started one of the world's first two bird ringing schemes in 1909 (they merged in the late 1930s), transferring responsibility, in 1937, to the British Trust for Ornithology (BTO), who continue to run it.

He was made an Honorary Fellow of the American Ornithologists' Union in 1928[7] and was awarded the Godman-Salvin Medal by the BOU in 1937.