According to the U.S. Census Bureau, the county has a total area of, of which is land and is water. It is the smallest county in Illinois by area.
Climate and weather
In recent years, average temperatures in the county seat of Hennepin 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 June 1988. Average monthly precipitation ranged from in January to in August.
As of the 2010 census, there were 6,006 people, 2,509 households, and 1,734 families residing in the county. The population density was. There were 3,074 housing units at an average density of. The racial makeup of the county was 96.6% white, 0.5% black or African American, 0.2% Asian, 0.1% American Indian, 1.4% from other races, and 1.1% from two or more races. Those of Hispanic or Latino origin made up 4.2% of the population. In terms of ancestry, 46.5% were German, 16.7% were Irish, 16.3% were Italian, 10.9% were Polish, 10.1% were English, 5.1% were Swedish, and 3.5% were American. Of the 2,509 households, 27.7% had children under the age of 18 living with them, 57.6% were married couplesliving together, 7.2% had a female householder with no husband present, 30.9% were non-families, and 26.0% of all households were made up of individuals. The average household size was 2.39 and the average family size was 2.85. The median age was 45.2 years. The median income for a household in the county was $56,458 and the median income for a family was $68,875. Males had a median income of $50,205 versus $31,886 for females. The per capita income for the county was $27,004. About 6.2% of families and 10.9% of the population were below the poverty line, including 21.8% of those under age 18 and 5.4% of those age 65 or over.
Prior to 1988, Putnam County was a Republican Party stronghold in presidential elections, backing the Republican candidate in all but three elections from 1892 to 1984. From 1988 to 2012, the county consistently backed Democratic Party presidential candidates, but none fared better than Illinois resident Barack Obama's 56.9% in 2008. However, he failed to win a majority of the county's votes four years later despite winning it overall in his 2012 reelection bid. This foreshadowed what was to come in the 2016 election, as the county swung 21.8 points Republican to back Donald Trump over Illinois-born Hillary Clinton by a margin of 19.9%.
