It is often said that Idaho County is not only the largest (in area) in the state, but also fourth largest in the nation. Now Idaho County is large: at 8502 square miles, it is roughly the size of New Jersey. (Yet it has only one intersection with a traffic light!)
But it is not even nearly the fourth largest county in the nation. The Alaska North Slope Borough covers 88, 817 square miles, and in fact, six of Alaska's boroughs are larger than Idaho County. Even if we confine ourselves to counties in the lower 48 states, there is San
Bernardino County in California, containing 20,105 square miles; Coconino County, Arizona, with 18,661 square miles; Nye County, Nevada, with 18,159 square miles; Elko County, Nevada, with 17,203 square miles; and so on down the list.
If we count Alaska's boroughs, Idaho County is the 24th largest county in the U.S. Among counties in the lower 48 states, Idaho County is number 18 in size.  
*Contributed by: Bill Salmon

The area now comprising Idaho County was part of Oregon Territory from 1848 to 1859. With Oregon statehood, it became a part of Washington Territory, and, in 1863, of Idaho Territory. The mining town of Florence was named the first County Seat of Idaho County December 20, 1861. After the decline in population of Florence, Warren became the second County Seat June 1, 1869.  June 14, 1875 there was a special election, and Mt. Idaho, which started as a way station along the trail to Florence, was elected the third County Seat.  November 4, 1902, Grangeville became the fourth County Seat, and still is today.