Born in York, Snow was educated at a private school before becoming apprenticed to a surgeon in Newcastle upon Tyne. He then became an assistant apothecary, treating patients during the 1832 cholera epidemic, before attending the Great Windmill Street School of Medicine in London. He qualified MRCS in 1837 and LSA in 1838 and became a general practitioner in Soho where he was an active member of the Westminster Medical Society.
In 1846, he watched James Robinson administer ether and realised successful anaesthesia depended on controlling the temperature at which the ether vaporized. After three months of experiments he established the scientific principles for inhalational anaesthesia and designed safe and effective inhalers for ether and chloroform. His monograph On the Inhalation of the Vapour of Ether was published in 1847.
Snow worked at St. George’s, King’s, and University College Hospitals, quickly becoming the leading specialist anaesthetist and building up a substantial practice. He discovered cholera was a water-borne disease following his observations during the epidemics of 1848 and 1854 but this was slow to be accepted. In 1853 and 1857 he administered chloroform to Queen Victoria during childbirth. He died following a stroke and is buried in the Brompton Cemetery.