Giles Mompesson (1583/1584 – 1663) was an English office holder and politician who sat in the House of Commons between 1614 and 1621 when he was sentenced for corruption. He was officially a "notorious criminal" whose career was one based on speculation and graft. His name came to be regarded as a synonym for graft and official corruption because he used nepotism to gain positions for licensing businesses by which he pocketed the fees. In the reaction against Charles I, Mompesson's name was invoked as a symbol of all that was wrong with . Sir Giles Overreach, the anti-hero of Philip Massinger's 1625 play A New Way to Pay Old Debts, is based on Mompesson.
Mompesson was born in Wiltshire. He grew up into a small, swarthy individual with black hair. He entered Hart Hall, Oxford in 1600, but left without a degree the next year for Lincoln's Inn; he later departed there without becoming a lawyer. In 1606 or 1607, he married Katherine, the daughter of Sir John St. John, one of the most prominent men in Wiltshire. Through his father-in-law's influence, Mompesson became a Member of Parliament for Great Bedwyn in 1614. Another daughter of John St. John (and thus Mompesson's sister-in-law) married Edward Villiers, the half-brother of George Villiers, and Mompesson's connection to George Villiers was the key to his later despotism. George Villiers became James I's favourite (and alleged lover), rising to the rank of Duke of Buckingham by 1616, and Mompesson was quick to use his family connections. His infamous career was tied directly to that of George Villiers and James I.
In 1616, Mompesson used his influence to propose to Villiers that there be a commissioner of inns. Justices of the peace were in charge of granting licences to taverns, but there was ambiguity about inns. While Fulke Greville, the Chancellor of the Exchequer, disapproved of the legal standing of Mompesson's plan, Sir Francis Bacon, then the Attorney General, felt that it was legal, and Villiers urged the king to pressure the exchequer to approve the plan. Mompesson therefore became one of three commissioners of inns in 1617. The fines that he could levy against inns out of compliance was left to his discretion, and the fees that he could charge to license an inn were similarly up to his own judgment. His only constraint was that four-fifths of the money go to the Treasury. On 18 November 1616, James knighted Mompesson to give him more authority when dealing with innkeepers.