[1] Previous county seats of the area were Florence (1864–68), Washington (1868–75), and Mount Idaho (1875–1902).

[2] Discovery of gold occurred in succession at Elk City, Newsome, and Florence during the spring and summer of 1861.

The Union affiliated miners on the northern edge of the settlement named their portion of the settlement Washington while the Confederate affiliated miners named their portion Richmond.

Out of all these settlements, only Cottonwood went on to eventually become one of Idaho County's seven incorporated cities.

Population data was returned for both years for Warrens (660/521), Florence (575/254), Elk City (372/219), Slate Creek (216/117), Clearwater Station (212/76), and Newsome (62/24).

For 1864, data was also returned for the settlements of Mount Idaho (74), Miller's Camp (36), and Cottonwood (17).

[8][9] Settlement at White Bird occurred some time prior to 1870 as a precinct under the same name is listed with 71 inhabitants at the 1870 census.

Efforts to force White Bird's band of Nez Perce tribesmen to the Nez Perce Reservation led to the Battle of White Bird Canyon in 1877.

[13] Ferdinand and Kooskia were settled starting in 1895 and along with Cottonwood and Stites, were all incorporated prior to 1920.

Boise was partitioned off in January 1863 with the Payette River being the primary dividing line.

Three boundary adjustments were made with Nez Perce and Ada between 1866 and 1867 and Lemhi was created in 1869 from territory east of the junction of the Middle Fork and main Salmon Rivers.

The southern border began to take shape after two changes in 1891 and 1895 exchanged territory between Washington and Idaho counties.

The southeast portion of the Nez Perce Indian Reservation is in the county's northwest corner.

As of the 2010 United States Census, there were 16,267 people, 6,834 households, and 4,536 families living in the county.

Idaho County has consistently supported the Republican candidate in national elections.

A Democratic candidate has not carried the county since 1964, when Lyndon B. Johnson won in a national landslide.

Since then, the last Democratic candidate to crack 40% of the county's vote was Jimmy Carter in 1976.