According to the U.S. Census Bureau, the county has a total area of, of which is land and is water. The Mississippi Palisades State Park is in this county, just north of the city of Savanna. The Savanna Army Depot is located partly in this county.
In recent years, average temperatures in the county seat of Mount Carroll have ranged from a low of in January to a high of in July, although a record low of was recorded in January 1910 and a record high of was recorded in July 1936. Average monthly precipitation ranged from in January to in June.
Demographics
As of the 2010 United States Census, there were 15,387 people, 6,622 households, and 4,343 families residing in the county. The population density was. There were 8,437 housing units at an average density of. The racial makeup of the county was 96.9% white, 0.8% black or African American, 0.3% Asian, 0.3% American Indian, 0.6% from other races, and 1.1% from two or more races. Those of Hispanic or Latino origin made up 2.8% of the population. In terms of ancestry, 40.4% were German, 14.0% were Irish, 11.2% were English, and 10.6% were American. Of the 6,622 households, 26.3% had children under the age of 18 living with them, 53.1% were married couples living together, 8.2% had a female householder with no husband present, 34.4% were non-families, and 29.8% of all households were made up of individuals. The average household size was 2.29 and the average family size was 2.80. The median age was 46.5 years. The median income for a household in the county was $44,805 and the median income for a family was $55,341. Males had a median income of $42,421 versus $27,552 for females. The per capita income for the county was $25,914. About 7.8% of families and 11.7% of the population were below the poverty line, including 18.4% of those under age 18 and 5.8% of those age 65 or over.
As a part of Yankee-settled Northern Illinois, Carroll County became solidly Republican upon that party's formation in the 1850s. Of all the counties won by inaugural Republican Party presidential nominee John Charles Frémontin 1856, Carroll County was to maintain the longest unbroken string of supporting the GOP in subsequent elections. It would give a plurality to every subsequent Republican Presidential nominee up to George W. Bush in 2004, beating by three elections the second longest run of Indiana's Porter County which was to give a plurality to Bill Clintonin 1996. In that 1996 election Bob Dole won Carroll County by only 1.51 percentage points – the smallest margin by a Republican to that point – and in 2008 Illinois native Barack Obama broke this last remaining GOP streak stretching back to Frémont by carrying the county by 4.80 percentage points. Obama was to repeat his win in 2012 by 1.49 percent, but a dramatic swing to Republican Donald Trump in 2016 saw him win by the largest margin since Ronald Reagan’s 1984 landslide by gaining 59.6% of the vote in Carroll County, scoring slightly higher than George H.W. Bush's victory in 1988 in which Bush won 59.4% of the vote in Carroll County. The largest margin of victory ever in Carroll County was achieved by Warren G. Harding who won 86.7% of votes in the county during the 1920 United States presidential election.
