[3] Fagan was born at Gonalston Hall, Nottinghamshire, England.

Fagan's parents were Lt. Col. George Haliburton Foster Peel Vere-Laurie and Caroline Judith Francklin.

[4] She was married on 21 October 1960 to Captain Christopher Tarleton Feltrim Fagan, son of Christopher Frederick Feltrim Fagan and Helen Maud Tarleton,[4][5] with whom she has had two sons: Christopher Hugh Tarleton Feltrim Fagan (died in a motor car accident in 1987) and James Tarleton Feltrim Fagan.

[5] Lady Mary is chair of trustees of the Countess of Brecknock Hospice Trust.

She is also a trustee of the Overlord Embroidery Trust, The Edwina Mountbatten and Leonora Children's Foundation, and Winchester Cathedral Trust.