William Herbert, 1st Marquess of Powis

William Herbert, 1st Marquess of Powis, PC (1626 – June 2 1696) was an English nobleman.

He succeeded his father, the 2nd Baron Powis, as 3rd Baron Powis in 1667, and was created Earl of Powis in 1674 by King Charles II and Viscount Montgomery, of the Town of Montgomery, and Marquess of Powis in 1687 by King James II, having been appointed to the Privy Council in 1686. He married in July 1654, Lady Elizabeth Somerset (c.1633–1691), daughter of Edward Somerset, 2nd Marquess of Worcester (d. 1667), by whom he had six children, a son and heir and six daughters, one of whom, Winifred, married William Maxwell, 5th Earl of Nithsdale, who was condemned to death for high treason for participating in the Jacobite Rising of 1715. Lady Nithsdale famously organised her husband's escape from the Tower of London.

A cousin of the 1st Baron Herbert of Cherbury, Powis was, together with his wife, one of the leading Roman Catholics, and remained faithful to the deposed King James after the Glorious Revolution of 1688. It was he who spirited away Queen Mary and the infant James, Prince of Wales, and took them into their French exile. As a reward, he was created "Duke of Powis" and "Marquess of Montgomery" in the Jacobite Peerage by King James.

In 1690 Powis landed in Ireland with James, where he acted as one of his principal advisers. James appointed him to his Irish Privy Council and made him Lord Chamberlain. He remained in Ireland until the king's flight back to France after the Battle of the Boyne, and settled again at the exiled Jacobite Court at St Germain. Powis was a prominent figure in the Jacobite Court, serving as Lord Steward and Lord Chamberlain of the household, but he seems to have been rather marginal in the king's counsels. His wife continued as Principal Lady of the Bedchamber to Queen Mary of Modena and Royal Governess to James, Prince of Wales until her death on 11 March 1691. James made Powis a Knight of the Garter in April 1692. Nevertheless others exercised more influence at Court as Powis struggled to maintain the dignity of a royal household on an insufficient income. Having lost estates valued at £10,000 a year, he had given up more than most for the Jacobite cause. He died, aged about seventy, on 12 July 1696, after a riding accident in St Germain, and was buried there the next day.

He was succeeded by his only son William Herbert, Viscount Montgomery (Jacobite Marquess of Montgomery) as second Marquess of Powis and Jacobite second Duke of Powis) (1665–1745), who was later jailed in the Tower as a Jacobite and fought a long battle in the courts to retain some of his property, resulting in the restoration of his family's estates. He was relieved of the attainder placed on his father and was restored to the forfeited peerages in the rank of marquess in 1722.