In recent years, average temperatures in the county seat of Lacon have ranged from a low of 15 °F (−9 °C) in January to a high of 87 °F (31 °C) in July, although a record low of −27 °F (−33 °C) was recorded in January 1999 and a record high of 103 °F (39 °C) was recorded in July 2005. Average monthly precipitation ranged from 1.55 inches (39 mm) in January to 4.20 inches (107 mm) in May.
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/km2). There were 5,914 housing units at an average density of 15.3 per square mile (5.9/km2). 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.