Washington County, ID adjoins Payette near Weiser metropolitan area granting a 2020 estimated population standing at 12,921 making it the 11th highest populous county located in Idaho overall. Its county seat is Weiser with other cities include Cambridge & Midvale-Weiser itself equates out to Washington's largestcity aside from noted country seat. This area had been formed from Idaho and Lewis Counties back in 1879 when local citizens petitioned for new administrative boundaries - posthumously named after President George Washington effrontery period across Rutherford B Hayes .