She married the Earl of Waldegrave in 1759. He died in 1763. Being illegitimate and non-royal, she secretly married the Duke of Gloucester, nephew of King George III. The Duchess of Devonshire's Gossip Guide to the Eighteenth Century had a good article about her here that states, "George threw a temper tantrum and tried to prove the marriage to be a farce but was defeated. The two were recognized but not welcomed at court, forcing them into exile onto the continent." Her Wikipedia article is here.

ca. 1763 Maria Walpole, Countess of Waldegrave, later Duchess of Gloucester in a black mourning dress by Thomas Gainsborough (Dunedin Public Art Gallery - Dunedin, Otego Region, New Zealand)
