Sir William Henry Allchin was born on October 16, 1846, in Paris, France.

He was an English physician and lecturer on comparative anatomy, physiology, pathology, and medicine. He studied medicine at the University College, London (M.B., 1871). He became an assistant physician at Westminster Hospital in 1873, a physician in 1877, dean from 1878 to 1883 and again from 1890 to 1893. He lectured on comparative anatomy at University College and on pathology (1873-1878), physiology (1878-1882) and medicine (1882-1892) at the Westminster Hospital. He retired from the hospital staff in 1905. Allchin was Senior Censor of the Royal College of Physicians and delivered the Bradshaw Lecture in 1891, the Harveian Oration in 1903 and the Lumleian Lectures in 1905. He was the editor of the Manual of Medicine and a contributor to Quain’s Dictionary of Medicine, Allbutt’s System of Medicine, and Keating's Cyclopaedia of the Diseases of Children. In 1901, he served as President of the Medical Society of London. He was knighted in 1907, and in 1910, he became Physician-Extraordinary to King George V.

In 1880, he married Margaret Holland (1853–1934). He died on February 8, 1912, in East Malling, Kent, England.