The plot of Mary Barton primarily focuses on the Barton family, particularly Mary and John Barton, the novel’s protagonists. Mary and John are influenced by wider family members who are mainly absent from the plot such as Mary Barton Sr., Tom, the unnamed younger son who dies during the novel, and Esther. Through these relationships, the novel explores family support networks, duty, and loss, especially in the context of economic hardship.

Mary Barton is a dynamic character who undergoes great change throughout the novel. From early in her life, Mary “​​dwelt upon and enjoyed the idea of some day becoming a lady, and doing all the elegant nothings appertaining to ladyhood” (93), following in the example that she thinks has been set by her aunt Esther. Mary’s character development and moral decision-making revolves around her gradual movement away from this aspiration. Mary finds herself taking risks with the wealthy and handsome Harry Carson, whom she believes wants to marry her and give her and her father a better life. Mary’s hopes and intentions are pure, but she is often guided by her own self-interest and the image of the life she believes she will have in the future.