Mortain (today a town and sub-prefectural seat in Manche, France) was a county of Normandy during the Middle Ages. It was probably established around 1027, for Robert, a son of Duke Richard I.
At the Norman Conquest, the count was Robert, half-brother of William the Conqueror, who came to have extensive land-holdings in England as well.
English holders, after the Conquest included Stephen, nephew to Henry I and later king in his own right; Stephen's son William (who died childless); and John, brother to Richard I, who held the title during Richard's reign.
Mortain was lost, with Normandy, to the English during John's reign, but later, when the house of Lancaster reconquered the area, a grandson of John of Gaunt, Edmund Beaufort, was created count of Mortain, and bore the title until his elvation, in 1441, to the earldom of Dorset.