Mary Reilly acts as a companion piece to the original The Strange Case of Dr. Jekyll and Mr. Hyde. Told from the perspective of Jekyll's housemaid, it offers both a more feminist and class-based reading of the source material.

Obviously, by telling the story from a lower class, female perspective, author Valerie Martin is placing more emphasis on the severe class and gender divides in Victorian England. Though Mary is bright and literate, she has no real opportunity to move forward socially. She is essentially trapped doing domestic work, though she does so with little complaints.

As a woman, she must be careful of compromising her reputation, especially with the heady sexual tension that exists between herself and her charismatic employer. Mary is contrasted and compared with other female characters in the story, such as her long-suffering mother who was trapped in an abusive marriage and Mrs. Farraday, a "fallen woman" stuck at the lowest rungs of the social ladder and exploited by wealthy, upper class men who present respectable faces to the world.

The biggest theme of the novel is, like Stevenson's text, about the duality of humankind. Both Jekyll and Mary are interested in how both good and evil reside within every individual's heart. Jekyll is more disturbed by it and seeks to rid himself of all evil impulses in order to become a better man. As in the original story, this backfires, making him more of a monster.

Mary is the opposite of her master in that she accepts the duality of her heart. She knows she has baser impulses and evil thoughts, especially in her anger towards her abusive father. She admits that while she is uncomfortable with her dark side, it makes her a fuller person and resisting evil makes her virtue all the stronger from the conflict.