Mary was born in Hadham Parva, Hertfordshire, England sometime before 16 December 1630. She was the daughter of Sir Arthur Capell, 1st Baron Capell of Hadham and Elizabeth Morrison.
On 28 June 1648, Mary married her first husband Henry Seymour, Lord Beauchamp and they had one son and one daughter. Her husband was a Royalist, imprisoned during the English Civil War. Her second husband, whom she married on 17 August 1657 was Henry Somerset, who became 1st Duke of Beaufort, by whom she had six children.
She died on 7 January 1715 in Chelsea, London, England at the age of 84. She was buried in Badmintonton, Gloucestershire. Her many descendants include Diana, Princess of Wales, and the Dukes of Beaufort