In the novel They do it with Mirrors, Miss Moncrieff is a woman who looked after her invalid mother in St. Mary Mead. Miss Moncrieff is described as having been "a slave to a tyrannical invalid mother", but she longed to travel and see the world. When her mother died, she was free to embark on her travels. However, she got no further than Hyères, where she visited one of her mother's oldest friends, and was so moved by the plight of "an elderly hypochondriac", that she cancelled her travel reservations and settled down there.
Miss Marple thinks of Miss Moncrieff while she is talking to Carrie Louise about her daughter Mildred Strete, so Miss Moncrieff is probably a parallel of Mildred.
In the film adaptation Murder with Mirrors (1985), Miss Marple also has a village parallel for Mildred but the name has been changed to "Mary Alice Rineker". Like Miss Moncrieff, Mary had spent most of her life looking after an ailing father. When he died, the villagers thought she would at last be free to travel. However her first trip was to Cornwall where she met an elderly aunt who had recently become bed-ridden. Mary Alice offered to stay and help her and had stayed there ever since. To Miss Marple, Mary Alice was a parallel of Mildred, a person who was happiest when given an opportunity to help. In the adaptation, Carrie Louise took Miss Marple's advice and dismissed the housekeeper Juliet Bellever so that Mildred could take over that role.