Boundary County was formed in 1915 from Bonner County. It was named Boundary County because it lies on the border of Canada, Washington and Montana. Settlement of the area started with the establishment of Bonners Ferry on the Kootenai River in 1864. Settlement was limited to the ferry operation until about 1890. The town of Bonners Ferry was established in 1893. At that point settlement was still sparse with small ranching and mining operations, but an expanding timber economy. The population was 9,871 at the 2000 census; it was estimated at 10,972 in 2010.

The county seat and largest city is Bonners Ferry.