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 through its service organisations, including St John Ambulance and St John Eye Hospital Group, the memberships and work of which are not constricted by denomination or religion. It 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.
In 1823, the Council of the French Langues—a French state-backed and hosted faction of the Order of Malta (Sovereign Military Hospitaller Order of St John of Jerusalem of Rhodes and of Malta)—sought to raise through private subscription sufficient money to restore a territorial base for the Order of Malta and aid the Greek War of Independence. This was to be achieved by issuing bonds in London to form a mercenary army of demobilized British soldiers using readily available, cheap war surplus. A deal transferring various islands to the Order of Malta, including Rhodes when captured, was struck with the Greek rebels, but, ultimately, the attempt to raise money failed when details leaked to the press, the French monarchy withdrew its backing of the council, and the bankers refused the loan.
The council was reorganised and the Marquis de Sainte-Croix du Molay (previously number two of the council and a former Order of Malta administrator in Spain) became its head. In June 1826, a second attempt was made to raise money to restore a Mediterranean homeland for the order when Philippe de Castellane, a French Knight of Malta, was appointed by the council to negotiate with supportive persons in Britain. Scotsman Donald Currie was in 1827 given the authority to raise £240,000. Anyone who subscribed to the project and all commissioned officers of the mercenary army were offered the opportunity of being appointed knights of the order. Few donations were attracted, though, and the Greek War of Independence was won without the help of the knights of the Council of the French Langues. De Castellane and Currie were then allowed by the French Council to form the Council of the English Langue, which was inaugurated on 12 January 1831, under the executive control of Alejandro, conde de Mortara, a Spanish aristocrat. It was headquartered at what Mortara called the "Auberge of St John", St John's Gate, Clerkenwell. This was the Old Jerusalem Tavern, a public house occupying what had once been a gatehouse to the ancient Clerkenwell Priory, the medieval Grand Priory of the Knights Hospitaller, otherwise known as the Knights of Saint John. The creation of the langue has been regarded either as a revival of the Knights Hospitaller or the establishment of a new order.
The Reverend Sir Robert Peat, the absentee perpetual curate of St Lawrence, Brentford, in Middlesex, and one of the many former chaplains to Prince George (Prince Regent and later King George IV), had been recruited by the council as a member of the society in 1830. On 29 January 1831, in the presence of Philip de Castellane and the Agent-General of the French Langues, Peat was elected Prior ad interim. He and other British members of the organisation, with the backing of the Council of the French Langues, then, on the grounds that he had been selling knighthoods, expelled Mortara, leading to two competing English chivalric groups between early 1832 and Mortara's disappearance in 1837. On 24 February 1834, Peat, three years after becoming prior ad interim, in order to publicly reaffirm his claim to the office of prior and in the hope of reviving a charter of Queen Mary I dealing with the original English branch of the Order of Malta, took the oath de fideli administratione in the Court of the King's Bench, before the Lord Chief Justice. Peat was thus credited as being the first grand prior of the association, however, "W.B.H." wrote in January 1919 to the journal Notes & Queries: "His name is not in the knights' lists, and he was never 'Prior in the Sovereign Order of St. John of Jerusalem': he became an ordinary member of that Order on Nov. 11, 1830."