One of the three regions that Arnor was broken into in the year 861 of the Third Age of the Sun. The other two were Arthedain and Rhudar.
The Dúnedain of Cardolan fought the forces of the Witch-King of Angmar until they were forced to flee to the Old Forest and the Barrow-Downs in the year 1409. They were able to return soon after and the Dúnedain inhabited the country again until the Great Plague of 1636. After that the region was no longer considered to be a country. However, there were still settlements in existence until the War of the Ring and perhaps after.
Cardolan was the region between the Baranduin and the Gwathlo-Mitheithel and south of the East Road.