Mary Loomis was the daughter of Lila and Sam Loomis. She grew up hearing horrible stories from her mother about Norman Bates and how Bates had killed Mary's aunt, Marion Crane. As a young woman, Mary became a psychology student.
After Norman Bates was released, Lila Loomis convinced Mary to help her make Norman go mad once again. Mary took a job as a waitress at Statler's Diner. She worked there for four days before Norman also started working there. In that time she said she had broken a dozen dishes.
Mary may or may not have had a boyfriend named Scott. She appeared to argue with him loudly on the telephone. This prompted Norman to invite her to stay with him when it was made clear that Scott had dumped Mary. Mary agreed and she began living at the Bates House.
Though nervous at first, Mary began to enjoy Norman's company. She believed he was no longer the man he had been when he killed her aunt. She decided to stop helping her mother with the conspiracy to make Norman go mad. She even provided Norman with an alibi when Sheriff John Hunt questioned him with respect to the disappearance of a local boy.
Mary accidentally killed Dr. Raymond in the Bates House. She also stabbed Norman several times as he confronted her about it. When Mary saw her mother's body in Norman's cellar she changed her mind about Norman, now believing him to be a killer after all (unaware that it was really his aunt, Emma Spool, who killed her mother) She was about to stab Norman to death when Sheriff Hunt and Deputy Pool burst upon the scene. Pool shot and killed Mary to save Norman's life.