Maria Mainwaring is the sister of Mr. Mainwaring, who lives in Langford with his wife Mrs. Mainwaring. She is about twenty-five years old and on the catch for a husband. Her brother was poor before his marriage, and the expense of a season in London impoverishes her for two years, which suggests she must also need to marry well due to a lack of funds.
She hopes to marry Sir James Martin, but when Lady Susan Vernon comes to Langford, she charms Sir James into proposing to her daughter Frederica instead. Maria is incensed, and she and her sister-in-law (who is jealous of Lady Susan's flirtation with her husband) unite to drive her out of Langford. Maria later visits her aunt in London to recapture Sir James, only to impoverish herself for nothing when he marries Lady Susan herself.