Established February 7, 1889 with its county seat at Rocky Bar. Named for the Ida Elmore mines, the area's greatest silver and gold producer of the 1860s. The Oregon Trail crossed the Snake River at Three Island Crossing near Glenns Ferry. A station on the overland stage route, originally named Rattlesnake, was moved to the railroad line and became Mountain Home. On February 4, 1891 the county seat was moved to Mountain Home.