According to the U.S. Census Bureau, the county has a total area of, of which is land and is water. The county is distinctly bisected by the Illinois River, splitting the county into two uneven sections.
Climate and weather
In recent years, average temperatures in the county seat of Lacon have ranged from a low of in January to a high of in July, although a record low of was recorded in January 1999 and a record high of was recorded in July 2005. Average monthly precipitation ranged from in January to in May.
As of the 2010 United States Census, there were 12,640 people, 5,161 households, and 3,549 families living in the county. The population density was. There were 5,914 housing units at an average density of. The racial makeup of the county was 97.1% white, 0.4% Asian, 0.3% black or African American, 0.2% American Indian, 1.1% from other races, and 0.9% from two or more races. Those of Hispanic or Latino origin made up 2.5% of the population. In terms of ancestry, 43.0% were German, 16.7% were Irish, 14.4% were English, 7.2% were Italian, 6.2% were American, and 6.1% were Polish. Of the 5,161 households, 27.4% had children under the age of 18 living with them, 56.7% were married couples living together, 7.7% had a female householder with no husband present, 31.2% were non-families, and 26.8% of all households were made up of individuals. The average household size was 2.40 and the average family size was 2.88. The median age was 44.8 years. The median income for a household in the county was $49,116 and the median income for a family was $64,781. Males had a median income of $46,793 versus $28,549 for females. The per capita income for the county was $24,991. About 6.8% of families and 9.5% of the population were below the poverty line, including 11.4% of those under age 18 and 7.0% of those age 65 or over.
In its early days Marshall County was a swing county, voting for winning Whig candidate William Henry Harrison in 1840 but otherwise supporting the Democratic Party until 1852. Its reputation as a swing county was to be sustained with the growth of the Republican Party: it voted for the winning candidate in every election from 1852 to 1912 except 1884 and 1888. Since World War I, Marshall has generally been a strongly Republican county. Only two Democrats – Franklin D. Roosevelt in 1932 and 1936 plus Lyndon Johnson in 1964 – have gained an absolute majority in Marshall County over the past twenty-six elections, although Bill Clinton won pluralities in both his elections.
