Mary, the eldest daughter of Mrs. Crole and Lord Egremont, was brought up in her father's household at Petworth with the children of Elizabeth Ilive. This led to her being thought to be the daughter of Elizabeth Ilive, an error repeated in the "Complete Peerage". Lord Egremont's settlement and the entry of her marriage in the register of St. George Hanover Square, where she is called Mary Wyndham Fox, make it clear that she was the daughter of Elizabeth Fox.
On 18 October 1819 she married George FitzClarence, later created Earl of Munster, the eldest son of the future King William IV and Mrs. Jordan, and they had four sons and three daughters.
Her husband had always been an unhappy character; his resentment that his irregular birth had denied him his true rights had been exacerbated, according to Greville, by "the domestic unhappiness of a dawdling, ill-conditioned, vexatious wife". He had been supported financially to a large extend by his father-in-law Lord Egremont who seems to have felt a special affection for Mary.
On 20 March 1842 the Earl of Munster committed suicide and Mary died soon after her husband, on 3 December 1842 at the age of 50 and was buried with him in the nave of Hampton church
 

Source: Leo van de Pas.