His parents were Lawrence Bengston Bancks of Stockholm, commissioner of customs, and his wife Christina. He came to England in 1681 as a diplomat; he was secretary to the Swedish resident of the time in London, who was his uncle. The resident's name, Johan Barckman Leijonberg, is usually Anglicised as James Barkman Leyenburg; it is also given as John Birkman, Count of Lezenburgh.
Naval officer
Bancks joined the Royal Navy in 1681. In 1690 he served at the Battle of Beachy Head, taking over from his wounded captain. Bancks himself had a commission as captain shortly after the battle; he commanded HMS Cambridgein September 1690. In the same year he bought Hall Place, Berkshire. As captain of HMS Phoenix in 1692, Bancks was off the coast of Spain when he was driven ashore on 12 April by a superior French naval force. The Phoenix was burnt, to prevent its capture. He was captain in HMS Carlisle in 1693. He was on half pay from 1696, or from the conclusion of the Treaty of Ryswick. He was knighted in 1698, as captain of HMS Russell, which he had commanded since 1696.
In politics
He married the widow Mary Luttrell in 1696, and represented Minehead as Member of Parliament from 1698, initially with Alexander Luttrell, brother of Francis, his wife's first husband. He was subsequently involved in the rougher side of the Whig-Tory factional strife. Bancks had George Rooke as stepson for a short period, since Rooke's second wife was Mary Luttrell, daughter of his wife by her first marriage. Bancks, Rooke and some others belonged to a gentleman's club, for which commemorative medals were struck in 1703 by the visiting Swedish medallist, Bengt Richter; another member who was an M.P. was Tanfield Vachell. A legal case resulted from the connection. After a quarrel with Rooke, William Colepeper claimed that an attempt, on behalf of Rooke, was made upon his life. He had been assaulted at Windsor Castle in July 1703, by Bancks in particular, on the occasion of Colepeper's delivering a petition for Daniel Defoe who was imprisoned. After a trial before Lord-justice Sir John Holt, 14 February 1704, some persons associated with Rooke were fined for attempts to do Colepeper injury: Nathaniel Denew, John Merriam and Robert Britton. Later in 1704 Bancks was allowed the assistance of Sir Simon Harcourt the Solicitor-General to prepare against a case brought by Colepeper. In 1711 Bancks was attacked in an open letter, initially published anonymously, by the Whig publicist William Benson. It was provoked by an address the year before by Bancks to the borough, commending the doctrine of passive obedience over Whig resistance theory. Benson aimed to associate the "Minehead doctrine" he attributed to Bancks with the absolutism of Charles XII of Sweden. He followed it with another such letter. In 1713 Benson and James Milner of London stood against Bancks and Sir John Trevelyan, 2nd Baronet in Minehead. The Tory pair won the borough, but Bancks did not stand again.
Bancks was implicated in the "Gyllenberg Plot", a Jacobite conspiracy in 1716–7 set up by Carl Gyllenborg and Georg Heinrich von Görtz. He was taken into custody, with Charles Caesar, on 29 January 1717, the day on which General George Wade implicated Gyllenborg in plotting by finding incriminating papers. Another arrest was Boyle Smith. Bancks and Caesar had in fact raised and sent to Sweden £18,000 to support a putative Jacobite invasion; but there was little intention in Sweden of spending it for that purpose.
, also a Member of Parliament, was his son. When the younger Jacob Bancks died intestate, a complex lawsuit arose, involving the Swedish side of the family.
