Mary Morstan is a small and petite woman, whose mother died shortly after her birth. Her father was captain of an Indian regiment and disappeared without a trace in 1878.

The circumstances under which the captain had disappeared remain very mysterious. The blonde and very pretty Mary works as a governess in the home of the elderly Mrs. Cecil Forrester.

After receiving a mysterious invitation, Mary Morsten contacts the private detective Sherlock Holmes in his apartment at 221b Baker Street, London in the second book by author Sir Arthur Conan Doyle, The Sign of Four. The aim Holmes’ commission is to clarify the circumstances under which her ??father disappeared in 1878.

Dr. Watson, who is naturally also present during the visit, falls head over heels in love with the pretty blonde. Although becoming closer during the course of the book, Watson still does not reveal his feelings for her.

The reason for this is that Mary is entitled to half of of a very large oriental treasure if the case is solved. But if Holmes does solve the case, the incredibly wealthy Mary Morstan would become unreachable for Dr. Watson.

In the course of the investigation, Holmes and Watson are on the trail of a man who has been illegally placed in the possession of that treasure. The two manage to find the chest in which the treasure is located.

However, Mary Morstan opens this chest only to find out that it is actually empty. The perpetrator had managed to dispose of its content shortly before his capture by Holmes and the police, whereby he had emptied it in the Thames, so that the treasure, or whatever was in the chest, is now lying on the bottom of the river.

Since Mary may now not call any treasure her own, Dr. Watson no longer considers the possibility of any existing obstacles and confesses his feelings to Mary. Since his love reciprocates his feelings, the couple marries in 1888. After the wedding, Dr. Watson moves out of Holmes' apartment to his own place where he would live with his wife, Mary, and shortly thereafter opens a medical practice.

In later novels, Mary Watson appears only rarely and as a relatively unimportant minor character.

After the death of his wife, Watson moves back into Baker Street apartment. The flat is vacant at this time, as Sherlock Holmes has seemingly died in his struggle with Professor Moriarty at the Reichenbach Falls.