The order traces its origins back to the Knights Hospitaller in the Middle Ages, which was later known as the Order of Malta. A faction of them emerged in France in the 1820s and moved to Britain in the early 1830s, where, after operating under a succession of grand priors and different names, it became associated with the founding in 1882 of the St John Ophthalmic Hospital near the old city of Jerusalem and the St John Ambulance Brigade in 1887.
The order is found throughout the Commonwealth of Nations, Hong Kong, the Republic of Ireland, and the United States of America, with the worldwide mission "to prevent and relieve sickness and injury, and to act to enhance the health and well-being of people anywhere in the world." The order's approximately 25 000 members, known as confrères, are mostly of the Protestant faith, though those of other Christian denominations or other religions are accepted into the order. Except via appointment to certain government or ecclesiastical offices in some realms, membership is by invitation only, and individuals may not petition for admission.
The Order of St John is perhaps best known for the health organizations it founded and continues to run, including St John Ambulance and St John Eye Hospital Group. As with the order, the memberships and work of these organizations are not constricted by denomination or religion. The order is a constituent member of the Alliance of the Orders of Saint John of Jerusalem. Its headquarters are in London and it is a registered charity under English law.