Mary Herbert, Marchioness of Powis (died 8 January 1724), formerly Mary Preston, was the wife of William Herbert, 2nd Marquess of Powis and the mother of William Herbert, 3rd Marquess of Powis. Mary was the eldest daughter and co-heir of Sir Thomas Preston, 3rd Baronet, of Furness and his wife, the former Mary Molyneux. She married the Marquess of Powis, then known as Viscount Montgomery because of his father's links with the deposed king James II and VII, in about 1695. Shortly afterwards he was imprisoned as a result of his Jacobite sympathies. He was restored to his title and estates and recalled to Parliament in 1722; at this time he regained possession of Powis Castle, which the family had lost as the result of an action during the English Civil War.