The Jersiais emerged in the late 10th century as rulers of the Kingdom of Arvor and achieved greater prominence with the 1078 Arvorian conquest of Prydain. Over the years they expanded outwards by a combination of diplomacy, inheritance and conquest, culminating in their acquisition of Lyonesse in 1328 after Henri V died childless. However, their lands were divided in 1413 by Gwilym IV and V between his three sons, and the three appanages of Prydain, Lyonesse and Galicia gradually drifted apart to face separate destinies.
The Jersiais Dynasty became extinct in the male line in 1499 with the death of Duke John II of Galicia. It was succeeded in Prydain and Eriu by the Kentians, in Lyonesse and Arvor by the Orléannais, and in Galicia by a period of direct rule under the Spanish crown.