English naturalist from Plymouth, the son of a solicitor. As a child he was interested in marine biology but studied medicine at St. Bartholomew's Hospital in London and at the University Edinburgh where he achieved his MD (1812). He subsequently returned to zoology and was appointed assistant librarian at the British Museum (1813) where he found much work to be done and built good contacts with the staff at the Paris Museum. He was promoted to Assistant Keeper of the Natural History Department of the museum but had a reputation for eccentricity; he named nine genera after 'Caroline' or anagarams of the name, supposedly a mistress.

After a comparatively short career he was obliged to resign (1822) because of a nervous breakdown, brought on by overwork or perhaps his increasingly vehement dislike of his former protégé J.E. Gray and colleague J.F. Stephens. Gray, who became Keeper of the Natural History Department (1940-) considered that "a very contributing cause was Leach's concentration on self sacrificing toyl for the museum". Leach moved to Italy where he stayed with a sister but later died of cholera in Palazzo San Sebastian, near Tortona. Though known mainly as an ornithologist or entomologist, Leach also made British plant collections with original material at BM.