Marquess of Powis was a title in the Peerage of England. It was created in 1687 for William Herbert, 1st Earl of Powis. He had already succeeded his father as third Baron Powis in 1667 and had been created Earl of Powis in the Peerage of England in 1674; Marquess of Powis and Viscount Montgomery in 1687. James II, following his exile made him Duke of Powis and Marquess of Montgomery, but these titles in the Jacobite Peerage (though used) were not recognised in England.
The title of Baron Powis was created in the Peerage of England in 1629 for William Herbert. He was the son of Sir Edward Herbert, second son of William Herbert, 1st Earl of Pembroke and Anne Parr. This Herbert family were thus members of a junior branch of the prominent Welsh family headed by the Earl of Pembroke. The peerages became extinct on the death of the first Marquess's grandson, the third Marquess, in 1748.
Barbara, daughter of Lord Edward Herbert, younger son of the second Marquess of Powis and brother of the third Marquess, married Henry Arthur Herbert, who was created Earl of Powis in 1748. They inherited his estates. See this title for more information.