Her parents were Sydney and Esther Davies and Mary was the eldest of their two children, having one younger sister. Sydney Davies was teaching at the Queen Elizabeth Grammar School in Carmarthen in south Wales when Mary was born but, when she was six years old, the family moved to Llandovery.
Ang kanyang mga magulang ay Sydney at Esther Davies at Maria ay ang pinakamatanda ng kanilang dalawang anak, pagkakaroon ng isang batang babae. Sydney Davies ay pagtuturo at ang Queen Elizabeth Grammar School sa Carmarthen sa timog Wales kapag Maria ay ipinanganak ngunit, kapag siya ay anim na taong gulang, ang pamilya inilipat sa Llandovery.