Daughter of Lt-Col. Frank Wigram Foley and Eva Mary FitzHardinge Milman, 16th Baroness Berkeley.
Born as Mary Lalle Foley, she changed her name legally to Hon.
She succeeded to the title of 17th Baroness Berkeley on 5 April 1967 when the abeyance of the barony was terminated in her favour.
She died unmarried at the age of 87, on which her title passed to nephew Anthony Gueterbock.
Her probate was sworn the next year at £878,833 (equivalent to about £2,300,000 in 2023); her home was Pickade Cottage, Great Kimble in the Chilterns.