 The county was formed from unorganized lands and parts of Cheyenne, Dewey (then named Rusk) and Stanley Counties.The county was never formally organized, and was attached to Stanley County for governmental purposes.In 1895, the county was renamed Armstrong in honor of Moses K. Armstrong, a pioneer in the territory who lobbied for territorial organization and later served in the Territorial Legislature and as a territorial delegate to the United States House of Representatives.