Thomas Cresap was known in Western Maryland as a "border ruffian" and in Pennsylvania as the "Maryland Monster". In the 1730's he tried to expand the borders of Maryland at the expense of Pennsylvania and Virginia by settling German immigrants into disputed areas and surveying the source of the Potomac River as far south as possible. Only in 1746, with the arrival of Mason and Dixon from England, was this dispute finally resolved.

Cresap also was a driving force in the Ohio Company, an enterprise that sought to open an important trade route to the west. He engaged in a lengthy dispute with George Washington over property in the Ohio Valley. Cresap died in 1787, but his name lives on in Cresaptown, upstream from Cumberland, MD.