The Counts de la Mark were among the most powerful and influential Westphalian lords in the Holy Roman Empire.

The east–west flowing Ruhr separated the county into two different regions: the northern, fertile lowlands of Hellweg Börde; and the southern hills of the Süder Uplands (Sauerland).

About 1198 Count Frederick I purchased the Mark Oberhof, a parish land (Feldmark) on the territory of the Edelherren of Rüdenberg, liensmen of the Cologne archbishop Philip von Heinsberg.

The nearby town of Hamm was founded by his son Adolf I, Count of the Mark in 1226, it soon became most important settlement of the county and was often used as residence.

As Brabant and its allies were victorious, the County of Mark gained supremacy in southern Westphalia and became independent of the Archbishopric of Cologne.

In 1509 the heir to the throne of Cleves-Mark John III the Peaceful married Maria, the daughter of Duke William IV of Berg and Jülich.

In 1511 he succeeded his father-in-law in Jülich-Berg and in 1521 his father in Cleves-Mark, resulting in the rule of almost all territories in present North Rhine-Westphalia in personal union, except for the ecclesiastical states.

A long dispute about the succession followed, before the territory of Mark together with Cleves and Ravensberg was granted to the Brandenburg Elector John Sigismund of Hohenzollern by the 1614 Treaty of Xanten (generally accepted in 1666).