Mary was the daughter of Ephraim "Bob" Gerrard, and his wife, Eliza, who lived at the Lodge at Hunterbury. As a child, she often played with Elinor Carlisle and Roderick Welman. According to Roderick, Mary had been a "scrawny kid, all legs and arms, with a lot of messy fair hair".
Mrs Welman had always been fond of Mary, and had paid for her schooling as well as various extra activities, such as piano and French lessons.
Shortly before the events of the novel, Mary had returned from Germany, where she had been an au pair for two years.
After Mrs Gerrard's death, Mary did not get on well at the Lodge, because she did not get along with her father. She spent a lot of time at the house, and read to Mrs Welman.
Mary was described as being a "lovely creature with a kind of wild-rose unreality about her". She had a "long delicate neck, pale golden hair lying close to her exquisitely shaped head in soft natural waves, and eyes of a deep vivid blue".
Roderick became infatuated with Mary, after meeting her at Hunterbury. However, she turned him down because he was engaged to Elinor at the time.
Mrs Welman died without making a will, but she had expressed a desire to make provision for Mary. So, Elinor decided to settle two thousand pounds on Mary, who intended to train as a masseuse.
Mary later made a will, following Nurse Hopkins' advice, leaving everything she had to Mary Riley, who was her mother Eliza's sister.
Mary went to London to train as a masseuse, but returned to Maidensford after her father died, as she needed to clear his things out of the Lodge at Hunterbury.
Mary was found unconscious in the morning room at Hunterbury, and she later died. It was found that a large dose of morphine had been taken shortly before death.
It was later revealed that Mary was the illegitimate daughter of Mrs. Welman and Sir Lewis Rycroft. Eliza had been lady's maid to Mrs Welman, and Mrs Welman had gone to Scotland, and taken Eliza with her. Mary was born at Ardlochrie. Mrs Welman had arranged that Eliza was to marry Bob Gerrard, and Bob was to think that the child was Eliza's. They were to live at the Lodge, and Mrs Welman would pay for Mary's education.