According to the US Census Bureau, the county has an area of, of which is land and is water.
Climate and weather
In recent years, average temperatures in the county seat of Petersburg have ranged from a low of in January to a high of in July, although a record low of was recorded in February 1905 and a record high of was recorded in July 1954. Average monthly precipitation ranged from in January to in May.
As of the 2010 United States Census, there were 12,705 people, 5,140 households, and 3,683 families living in the county. The population density was. There were 5,654 housing units at an average density of. The racial makeup of the county was 97.5% white, 0.6% black or African American, 0.3% American Indian, 0.2% Asian, 0.4% from other races, and 1.1% from two or more races. Those of Hispanic or Latino origin made up 1.0% of the population. In terms of ancestry, 32.5% were German, 20.8% were American, 15.6% were Irish, and 14.3% were English. Of the 5,140 households, 32.4% had children under the age of 18 living with them, 57.5% were married couplesliving together, 9.6% had a female householder with no husband present, 28.3% were non-families, and 24.3% of all households were made up of individuals. The average household size was 2.44 and the average family size was 2.87. The median age was 42.6 years. The median income for a household in the county was $56,230 and the median income for a family was $65,882. Males had a median income of $42,408 versus $36,735 for females. The per capita income for the county was $26,281. About 6.1% of families and 7.7% of the population were below the poverty line, including 10.4% of those under age 18 and 9.4% of those age 65 or over.
Illinois counties have the option of using or rejecting the township form of providing local services. In Illinois, only 16 of the state's 102 counties have opted out of the township system; Menard County is one of the sixteen. In the years before World War I, Menard was a German Catholic and thus Democratic county, opposed to the pietism of Yankee Protestant Northern Illinois. Theodore Roosevelt in 1904 was the only Republican to carry the county in this time span. However, resistance against Woodrow Wilson’s participatory attitude during World War I allowed Charles Evans Hughes to carry Menard County in 1916 by a mere four votes. Since then, as German hostility to Wilson hardened, Menard has become a strongly Republican county. The only Democrats who have carried Menard County since 1916 have been Franklin D. Roosevelt in 1932 and 1936, and Lyndon Johnson in 1964. Excluding the landslide losses of Hoover, Landon and Goldwater, George Bush senior in 1992 is the solitary Republican candidate since 1920 to fail to win a majority in Menard County. Illinois-bred Barack Obama is the only Democrat since 1980 to reach forty percent of Menard County’s vote, but over the last two elections a strong anti-Democratic swing – common to all of rural downstate Illinois – has taken place.
