[2] Gem County is part of the Boise, ID Metropolitan Statistical Area.

[3] Fur trappers were in the area as early as 1818, and Alexander Ross explored Squaw Creek in 1824.

Prospectors and miners moved through the county in 1862 en route to the gold rush in the Boise Basin around Idaho City, and by the next year irrigation began along the Payette River.

Picket's Corral, northeast of Emmett was the base of operations for a gang of horse thieves and "bogus dust peddlers" between 1862 and 1864.

[3] The Payette Vigilance Committee eliminated the gang, bringing safety for citizens of the area.

[7][8] Census data for the area shows Squaw Creek Precinct under Boise County with a population of 30 in 1870.

[11] At the 1910 Census, three Emmett area precincts contained a population of 2,601 while three Boise County precincts of Upper Squaw Creek, Lower Squaw Creek, and Pearl contained 1,069 residents.

[12] The Black Canyon Diversion Dam on the river was built in the early 1920s, east of Emmett.

As of the 2010 United States Census, there were 16,719 people, 6,495 households, and 4,611 families living in the county.

[25] As of 2016 the largest self-reported ancestries/ethnicities in Gem County, Idaho were: [26] Note: the US Census treats Hispanic/Latino as an ethnic category.

As of the 2020 United States census, there were 19,123 people, 6,683 households, and 4,585 families residing in the county.

The last time a Democratic presidential candidate carried the county was in 1964 by Lyndon B. Johnson.

[30] Residents are in the area (but not the taxation zone) for College of Western Idaho.