[2] Elmore County comprises the Mountain Home, ID Micropolitan Statistical Area, which is also included in the Boise-Mountain Home-Ontario, ID-OR Combined Statistical Area.

[3][4] While the Oregon Trail crossed the Snake River in Elmore County, at Three Island Crossing near Glenns Ferry, the significant early settlements of Elmore County were mining settlements located primarily in northern Elmore County surrounding the ghost town of Rocky Bar.

A small portion of Ada County was also annexed in 1948, bringing Elmore to its present boundary.

A station on the overland stage route, originally named Rattlesnake, was moved west to the railroad line and became Mountain Home.

Anderson Ranch Dam is east of Mountain Home on the South Fork of the Boise River.

[12] Elmore County's highest point is Snowyside Peak at 10,651 feet (3,246 m) above sea level, located in the northeast corner of the county in the Sawtooth Wilderness.

The lowest elevation in the county is the Snake River in the southwest corner, at less than 2,500 feet (760 m).

20.70% of all households were made up of individuals, and 6.40% had someone living alone who was 65 years of age or older.

As of the 2010 United States Census, there were 27,038 people, 10,140 households, and 7,135 families living in the county.

[22] School districts include:[25] Residents in a portion of the county are in the area (but not the taxation zone) for College of Western Idaho.

[26] In the remainder, it is in the area (but not the taxation zone) for College of Southern Idaho.