The county's boundaries changed several times during Idaho's territorial period. Owyhee County (Idaho's oldest) and a portion of Oneida County were carved from the southern and eastern portion of the county as it existed under Washington Territory in late December 1863 and January 1864. When Idaho Territory established the county in February 1864, it contained all of present Ada, Canyon, and Payette counties. It also included most of present Boise and Gem Counties, the southern half of Washington County, and small portions of Adams, Custer, Owyhee, and Valley counties.