Arthur Martell is the second oldest child of the former Prince of Dorne, Aeron Martell. He was raised at Sunspear and in the Water Gardens with his sisters and a younger Triston Dayne; the group became close friends during this time. He and his elder brother, Calon, often did not get along, and a sibling rivalry grew as they aged.
During the Targaryen Rebellion, a recently knighted Ser Arthur Martell rode north, fleeing Sunspear and his father and brother's decision to support House Targaryen. Instead, he declared for the Blackfyres, and his father exiled him for the decision. He has not returned home or contacted House Martell since the conclusion of the war.