The High Court of the Admiralty originated with Acts of Parliament of 27 Hen VIII, c4, and 28 Hen VIII, c15, designed to punish pirates and robbers at sea. Such cases had previously been heard before King and Council, in Chancery, before the Admiral or before Commissioners. The Admiral's court used civil law, which precluded the death sentence unless confession or an eye witness was available. The Henrician Acts remedied this by requiring common law trials before judges and juries in a court sitting by commission of oyer and terminer (and if necessary, gaol delivery) directed to the Admiral or his deputy.
The criminal jurisdiction thus assumed extended to English havens, to the Thames below London Bridge and to the high seas. Apart from piracy, defined as marine robbery or spoil, the court dealt with charges of treason, murder, mutiny, desertion, sodomy, insurance frauds and abetting (accessories on land became liable under an Act of 11 & 12 Wm III c7). Offences against the regulations governing the River Thames seem to have passed to the City of London about 1700. Foreign nationals might be involved; if for piracy, trial was by the law of nations, but they were as likely to appear in court for murder or flag disputes.
From its institution in 1535 until 1759 there was no regular session timetable: even when, by 32 Geo II, c 25, s 20, sessions were fixed for March and October, this soon proved inadequate. 18th century expansion had brought greater variety into court business, extending its staple diet of piracy and murder to treason, desertion, insurance and abetting offences. Treason had multiplied after 1688 owing to cases of Jacobitism and desertion, as extended to taking foreign service. So did insurance fraud, after a failed Act of 4 Geo I was rectified by another of 11 Geo I, c 29. By 39 Geo III, c 37, the court was allowed to convict for manslaughter rather than murder. Its oyer and terminer sessions ended in 1834 when the Central Criminal Court took over its jurisdiction, though the judge of Admiralty was included in its processes.
Ten years later, by 7 & 8 Vict, c 2, judges of assizes were allowed to try the offences in any county. Vice-Admiralty courts had been sanctioned by the Henrician Acts, and under warrant from the Admiral were at first held, for instance, in South Wales, Dorset, Hampshire, Cumberland and Devon; but from the early 18th century they were more likely to be held in the colonies, in America, the West Indies and Africa: appeal to the High Court from these was possible. In London the Court met for a century or so at the Guildhall or in Southwark (Orton Key or the Sessions House). From 1660 the Justice Hall in the Old Bailey became available, and after 1700 it was the sole venue. Prisoners were detained in the Marshalsea prison, though usually transferred to Newgate for trial at the Old Bailey, and capital punishments were carried out at Execution Dock, Wapping.
The commissions of oyer and terminer which led to Admiralty Court sessions were directed to the Admiral or commissioners of Admiralty, to the judge of the Court and to some common law judges and doctors of civil law, from among whom the Admiralty Court judges were recruited. Occasionally the names of the secretary of state or of peers were included, but the proceedings were in the hands of the judges. Court records were prepared by the Registrar, and orders enforced by the Marshal.