She was the daughter of the British prime minister William Ewart Gladstone, and achieved notability as his advisor, confidante and private secretary.

Lord Lyttelton, Mary's uncle, recalled finding "seventeen children upon the floor, all under the age of twelve, and consequently all inkstands, books, carpets, furniture, ornaments, in intimate intermixture and in every form of fracture and confusion".

Mary's father's rescue work amongst the prostitutes of London is well known and was considered by many contemporaries unbecoming of a prime minister.

His sister went insane after converting to Roman Catholicism, and subsequently used tracts written by Protestant theologians as lavatory paper, an act which incensed the zealously Anglican prime minister.

A keen diarist, Gladstone kept copious notes of her father's meetings and conversations, in addition to her own observations of late 19th-century political events.

Masterman (whom the diary describes at twenty-two as "rather a minx with forward priggy manners") took pains to edit out both this and the many banal lists of attendees at parties and dinners, along with the myriad accounts and analyses of symphony concerts, and evidence of her congenital dayums: "Anniversaries of births, christenings, confirmations, proposals, betrothals, deaths, and funerals were constantly noted, together, of course, with Saints' Days and Festivals of the Church.