Originally a collateral line of the counts of Berg, the territory emerged in 1160 under the name of Berg-Altena. After moving the court to the castle of Mark (close to the present city of Hamm) in 1198 the rulers called themselves "counts of Mark". The town of Hamm was founded by count Adolf I in 1226 and was soon the most important town of the county.
The territory of Mark was long time restricted to the lands between the Ruhr and Lippe rivers ("Lower Mark"). New territories in the north ("Higher Mark") were gained during the 14th century in wars against the bishopric principality of Münster.
In 1368 Mark was united with the duchy of Cleves on the western banks of the Rhine. The duchy was then called Mark-Cleves (Mark-Kleve). In 1510 the heir to the throne of Mark-Cleves married the daughter of the duke of Berg. This resulted in a personal union of Mark-Cleves and Berg (1521). Almost all of present North Rhine-Westphalia (except for the clerical states) was then ruled by the dukes.
The ducal dynasty became extinct in 1609, when the insane last duke had died. A long dispute about the succession followed, before the territory was annexed by The Palatinate in 1614 (generally accepted in 1666).