Mary Hay, 14th Countess of Erroll

Mary Hay, suo jure Countess of Erroll (died 19 August 1758), 18th Hereditary Lord High Constable and Knight Marischal of Scotland, and 14th Countess of Erroll, was the Senior Great Officer among the Royal Officers of Scotland and Chief of the King's Household in Scotland. She inherited these titles in 1717 on the death of her unmarried brother, Charles Hay, 13th Earl of Erroll.

She was the daughter of Sir John Hay Earl of Killour, 12th Earl of Erroll (grandson of Sir George Hay, the younger son of the seventh Earl) and his wife, Lady Anne Drummond, sister of the Jacobite Dukes Perth and Melfort. Like her brothers, Countess Anne was an active Jacobite and a secret agent of the exiled court of the Old Pretender, "James III and VIII", at Saint Germain-en-Laye.

Before August 1722, Mary Hay, Countess of Erroll, married Alexander Falconer, an advocate and son of Sir David Falconer, Lord President of the Court of Session. His elder brother, David, succeeded a distant cousin as 5th Lord Falconer of Halkerton. Later, her husband adopted the Hay surname.

As eldest sister and heir of their line, she succeeded to the dignities enjoyed by her brother under the nomination of 16 February 1674, and was served his heir on 6 February 1718. She had her claim as High Constable allowed at the coronation of King George II of Great Britain, although she was represented by a deputy.

In 1745 she raised an army of Buchan men for Prince Charles Edward Stuart. Making Slains Castle the chief centre for landing Jacobite secret agents, she had an implied understanding with the naval officer patrolling the coast of Buchan to let her know when his ship was passing off Slains. Once landed at Slains, they were hurried inland to another of her strongholds, Delgatie Castle, with its hidey-holes and secret passage.

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