In Letter XIX, Eliza tells Lucy that Major Sanford unexpectedly visits her while she is alone in the garden. Sanford immediately declares his feelings and tries to convince her not to marry Boyer. He argues that being married to a minister like Boyer would limit her freedom and force her into a more restricted, domestic life. Eliza admits that she doesn’t agree with Sanford, but she is still attracted to the way he speaks and presents his ideas. Sanford asks to remain in her life as a friend and companion, even if he cannot be her lover. Eliza tells him that Mrs. and General Richman have concerns about him because of his reputation. Sanford admits that he has lived a life of pleasure and made immoral or foolish choices, but he argues that he is not truly evil and asks Eliza to judge him by his present behavior rather than his past. When General Richman enters, Sanford acts completely polite and charming, making the situation seem innocent. After he leaves, Mrs. Richman warns Eliza that Sanford is trying to seduce and manipulate her. She compares him to Lovelace, the manipulative man from Clarissa, and tells Eliza not to be fooled by his flattering words or promises of pleasure. Mrs. Richman reminds her that she already has an understanding with Boyer and should follow sincerity and virtue instead. Eliza starts realizing that Sanford's presence may be dangerous and says she is thinking about returning to Lucy's circle partly so she can get away from him.

Sanford tells Charles that his relationship with Eliza is going well because she enjoys his company, accepts his flirting, and listens to his compliments, even though she has not openly said that she loves him. Sanford believes Eliza may think she can change or reform him, but he admits that he has become genuinely attached to her and might even become more respectable if it meant keeping her. However, he is frustrated that he has not completely won her heart and suspects she still favors Boyer. Sanford is deliberately trying to separate Eliza from Boyer by emphasizing how different their personalities and lifestyles are. He admits that he does not plan to marry Eliza, but he does not want her to marry Boyer either. Part of his motivation is jealousy and part is revenge against Eliza’s friends, especially because they disapprove of him. Most importantly, he openly considers the possibility of seducing Eliza, saying that she knows his bad character and should not be surprised if he acts according to it. He says he is currently enjoying her company and wants to keep her nearby now that he has bought property close to her mother’s home.