According to the US Census Bureau, the county has a total area of, of which is land and is water. The Portneuf River flows through the county, meeting the Snake River at the county's lowest point, its northwestern corner. Bonneville Peak, on the eastern border in the Portneuf Range, is the county's highest point at 9,271 feet ASL; on its western slopes is the Pebble Creekski area.
As of the 2000 United States Census, there were 75,565 people, 27,192 households, and 19,224 families in the county. The population density was 68 people per square mile. There were 29,102 housing units at an average density of 26 per square mile. The racial makeup of the county was 91.29% White, 0.59% Black or African American, 2.91% Native American, 0.99% Asian, 0.16% Pacific Islander, 2.08% from other races, and 1.98% from two or more races. 4.68% of the population were Hispanic or Latino of any race. 23.1% were of English, 14.2% German, 11.1% American and 7.4% Irish ancestry. There were 27,192 households out of which 36.50% had children under the age of 18 living with them, 56.70% were married couplesliving together, 10.00% had a female householder with no husband present, and 29.30% were non-families. 22.80% of all households were made up of individuals and 7.60% had someone living alone who was 65 years of age or older. The average household size was 2.69 and the average family size was 3.20. The county population contained 28.10% under the age of 18, 14.60% from 18 to 24, 27.20% from 25 to 44, 20.00% from 45 to 64, and 10.10% who were 65 years of age or older. The median age was 30 years. For every 100 females, there were 97.70 males. For every 100 females age 18 and over, there were 94.40 males. The median income for a household in the county was $36,683, and the median income for a family was $44,192. Males had a median income of $36,056 versus $23,595 for females. The per capita income for the county was $17,148. About 9.80% of families and 13.90% of the population were below the poverty line, including 15.60% of those under age 18 and 7.60% of those age 65 or over.
2010 census
As of the 2010 United States Census, there were 82,839 people, 30,682 households, and 20,836 families in the county. The population density was. There were 33,191 housing units at an average density of. The racial makeup of the county was 89.8% white, 3.2% American Indian, 1.3% Asian, 0.8% black or African American, 0.2% Pacific islander, 2.1% from other races, and 2.7% from two or more races. Those of Hispanic or Latino origin made up 6.7% of the population. In terms of ancestry, 23.4% were English, 16.9% were German, 9.3% were Irish, and 7.0% were American. Of the 30,682 households, 35.5% had children under the age of 18 living with them, 52.2% were married couples living together, 10.7% had a female householder with no husband present, 32.1% were non-families, and 24.9% of all households were made up of individuals. The average household size was 2.64 and the average family size was 3.17. The median age was 31.4 years. The median income for a household in the county was $44,848 and the median income for a family was $54,650. Males had a median income of $43,538 versus $28,870 for females. The per capita income for the county was $21,275. About 10.6% of families and 14.0% of the population were below the poverty line, including 16.6% of those under age 18 and 4.2% of those age 65 or over.
Government and politics
The last Democratic candidate for President to win the county was Lyndon B. Johnson in 1964, although subsequent Republican victories have been by smaller margins than those in neighboring counties. Similar to other Idaho counties, an elected three-member county commission heads the county government. Other elected officials include clerk, treasurer, sheriff, assessor, coroner, and prosecutor. Like other eastern Idaho counties, Bannock County has a significant Mormon population which strongly tends to vote Republican. However, a substantial trade union presence in the county - as well as the Idaho State University community - often gives Democrats an overall advantage, especially in local races. Bannock County routinely elects more Democrats than Republicans to county-level offices. At the state level Bannock County is located in Legislative Districts 28 and 29. Democrats currently control two of these six seats in the Idaho Legislature. In 2016, Republicans were able to win House Seat A in District 29. Idaho Department of Correction operates the Pocatello Women's Correctional Center in Pocatello and in Bannock County.
