She was a daughter of the 6th Earl Beauchamp and the wife of Lt.-Col.
Henry Hepburn-Stuart-Forbes-Trefusis (a son of the 20th Baron Clinton) in 1905 and had issue.
Lady Mary was a close friend and correspondent of her brother Lord Beauchamp.
She was a friend and promoter of the composer Edward Elgar and is thought to be commemorated anonymously in one of his Enigma Variations entitled "Romanza (***)".
She was the first Secretary of the English Folk Dance Society (founded 1911) and was for a time a collector of English folk dances.
Trefusis Hall in the EFDSS HQ, Cecil Sharp House, is named after her.
She was also Lady of the Bedchamber to Queen Mary (having joined her household while Mary was Princess of Wales), directed a number of choirs and was the joint founder of the Association of Competitive Festivals in 1904 (Mary Wakefield was the other founder).