It is not known when Fornost was supposed to have been founded, or when the kings of Arnor moved there from Annúminas; one possibility is that when Eärendur died in T.A. 861, and Arnor was divided into three kingdoms, Eärendur's eldest son Amlaith moved to Fornost to rule his kingdom of Arthedain.
In T.A. 1974, Arthedain was overrun by the forces of Angmar, they captured Fornost, and King Arvedui fled into the northern wastes and was lost in the Ice Bay of Forochel. In the following year, a fleet from Gondor led by Eärnur landed at Mithlond, fought the Witch-king of Angmar in the plains west of Fornost, and defeated him and his armies, although the Witch-king himself escaped.