Jenner was born at Chatham and educated at University College London. He became a member of the Royal College of Surgeons in 1837, and a fellow of the Royal College of Physicians in 1852, and in 1844 took the London M.D. In 1847 he began at the London Fever Hospital investigations into cases of continued fever. His research enabled him finally to make the distinction between typhus and typhoid on which his reputation as a pathologist principally rests. In 1849 he was appointed professor of pathological anatomy at University College, and also assistant physician to University College Hospital, where he afterwards became physician (1854-1876) and consulting physician (1879), besides holding similar appointments at other hospitals. He was president of the Royal College of Physicians (1881-1888); was elected a Fellow of the Royal Society in 1864, and received honorary degrees from the University of Oxford, the University of Cambridge, and the University of Edinburgh. In 1861 he was appointed Physician Extraordinary, and in 1862 Physician in Ordinary, to Queen Victoria, and in 1863 Physician in Ordinary to the Prince of Wales; he attended both the Prince Consort and the Prince of Wales in their attacks of typhoid fever. In 1868 he was created a baronet.

As a consultant, Sir William Jenner had a great reputation, and he left a large fortune when he died, at Bishop's Waltham, Hants, on 11 December 1898, having then retired from practice for eight years owing to failing health.

Photographed by Herbert Rose Barraud of 96, Gloucester Place, London.