Marshall County is a county located in the U.S. state of Illinois. According to the 2010 census, it had a population of 12,640. Its county seat is Lacon.
According to the U.S. Census Bureau, the county has a total area of 399 square miles (1,030 km), of which 387 square miles (1,000 km) is land and 12 square miles (31 km) (2.9%) is water.
As of the 2010 United States Census, there were 12,640 people, 5,161 households, and 3,549 families residing in the county. The population density was 32.7 inhabitants per square mile (12.6/km). There were 5,914 housing units at an average density of 15.3 per square mile (5.9/km). 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.