What is the name of Lady Mary's son?

Lady Mary's son is George Crawley, formally George Matthew Crawley. He is the child of Lady Mary Crawley and Matthew Crawley, and his birth occurs on the same day that his father dies in a car accident.

George becomes important to the Crawley family’s succession because he is the male heir to the earldom and Downton estate. Matthew’s inheritance had already changed the line of succession, and George’s birth establishes a new heir within the family. His mother, Mary, remains closely involved in protecting his interests while he is a minor.

The character’s first name is George, while “Crawley” is his family name. His full name also includes Matthew, honoring his father. This can cause confusion with Robert Crawley, George’s grandfather and the current Earl of Grantham during most of the television story.

George later appears in the Downton Abbey films as a young member of the family. He is also the elder half-brother of Caroline, Mary’s daughter from her later marriage, and a cousin of Sybbie and Marigold.