William Cavendish, 1st Duke of Newcastle upon Tyne, KG, KB, PC (c. 16 December 1593 – 25 December 1676), who after 1665 styled himself as Prince William Cavendish, was an English courtier and supporter of the arts. He was a renowned horse breeder, as well as being patron of the playwright Ben Jonson and the intellectual group known as the Welbeck Circle.

Despite spending the then enormous sum of £15,000 entertaining Charles I in 1634, he failed to gain a significant political post. In the early stages of the First English Civil War, he was appointed Royalist Captain-General in Northern England; he financed much of the war effort himself, later claiming this totalled in excess of £1,000,000. After the defeat at Marston Moor in July 1644, a battle fought against his advice, he went into exile in Europe.

He returned to England after the Stuart Restoration in 1660, and although created Duke of Newcastle in 1665, he remained on the fringes of the court and became critical of Charles II. He died in 1676 and was buried in Westminster Abbey.

William Cavendish was born at Handsworth, South Yorkshire, the eldest surviving son of Sir Charles Cavendish and Catherine Ogle, descended from the Barons Ogle. He was a grandson of Bess of Hardwick and courtier William Cavendish. He had a younger brother, Charles (1594–1654), and the two remained close friends throughout their lives.

In 1618, Cavendish married Elizabeth Howard (1599–1643), with whom he had five children – Jane (1621–1669), Charles (1626–1659), Elizabeth (1626–1663), Henry, 2nd Duke of Newcastle (1630–1691), and Frances. Encouraged by their father, Jane and Elizabeth became minor poets and writers. In 1645, he married Margaret Lucas, a natural philosopher and writer. With his help and support, she became a popular writer of plays, poetry, and fiction, and was known as "mad Madge" for her extravagant style and affected manner.

Created a Knight of the Bath in 1610, he was elected Member of Parliament for East Retford in the 1614 "Addled Parliament" and succeeded his father in 1617. Cavendish was created 'Viscount Mansfield' in 1620 and 'Earl of Newcastle-upon-Tyne' in 1628 but failed to achieve high office, despite spending the enormous sum of £15,000 entertaining Charles I in 1634. However, in 1638 he was appointed governor of the Prince of Wales, then made a Privy Counsellor. When the Bishops' Wars began in 1639, he provided the king with a loan of £10,000 and a troop of volunteer horse. He was appointed Gentleman of the Robes in 1641, but was implicated in the Army Plot, and withdrew for a time from the court.

As tension increased, both Charles and Parliament tried to secure key ports and weapons; an attempt by Newcastle to capture Hull in July failed. When Charles formally declared war in August, Newcastle was given command of the four northern counties, largely because he was willing to pay for his own troops. In November 1642, he advanced into Yorkshire, raised the siege of York, and forced Lord Fairfax to retire after attacking him at Tadcaster.

Fighting continued during the winter, as Newcastle tried to secure a landing place for an arms convoy organised by Henrietta Maria, who was in the Dutch Republic purchasing weapons. He had insufficient troops to hold the entire area, and Parliamentary forces under Lord Fairfax and his son Sir Thomas, retained key towns like Hull, and Leeds. In late February 1643, a convoy with Henrietta Maria and weapons landed at Bridlington, and was escorted to Oxford. This success, combined with a victory at Adwalton Moor near Leeds in June, caused Newcastle to be created 'Marquess of Newcastle-upon-Tyne'.