Mary Dering was the daughter of Sir Edward Dering, 5th Bt. (1711-1762) of Surrendon-Dering. She married Sir Robert D'Arcy Hildyard, 4th Bt. of Patrington, Winestead, who served as High Sheriff for Yorkshire in 1783. Cotes also painted Mary's sister-in-law, Deborah Winchester, Lady Dering, in 1767 (now in the Carnegie Institute, Pittsburgh). Lady Dering was the daughter of John Winchester, M.D., of Nethersole House, Barham Downs, Kent, and married Mary's brother, Sir Edward Dering, 6th Bt., in 1765.