Shoshone County was formed under the Territory of Washington on January 9, 1861. Washington Territory legislators established the county in anticipation of the gold rush that occurred after the discovery of gold at Pierce in October, 1860. Their location of the northern boundary at a line drawn due east from the mouth of the Clearwater River, unknowingly placed the emerging mining settlement at Pierce outside of the county's boundaries while residents of the new Mormon settlement at Franklin were unknowingly within the established boundaries. Regardless of the geographic reality, the county seat was at Pierce. Growth at Pierce was so rapid that Shoshone County boasted the largest vote of any county within Washington Territory at the territorial election of July 8, 1861. In less than a year, Shoshone County contained additional settlements at Lewiston, Elk City, Newsome, and Florence. On December 20, 1861, Nez Perce and Idaho counties were created from most of the original territory of Shoshone County. On the following day, Shoshone's boundaries were shifted northward, containing most of present-day Clearwater County and a portion of present-day Shoshone County.