According to the US Census Bureau, the county has a total area of, of which is land and is water. Macon County is primarily flat, as is most of the state and all of the surrounding counties, the result of geological activity during the Pleistocene epoch. During the Illinoian Stage of the Pleistocene, the Laurentide Ice Sheet covered about 85 percent of Illinois, including the Macon County area. The subsequent thaw of the region and retreat of the ice sheet left central Illinois with its present characteristic flat topography. Because of its central location, Macon County is often referred to as "The Heart of Illinois."
Climate and weather
In recent years, average temperatures in the county seat of Decatur 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 February to in July.
As of the 2010 United States Census, there were 110,768 people, 45,855 households, and 29,326 families residing in the county. The population density was. There were 50,475 housing units at an average density of. The racial makeup of the county was 79.3% white, 16.3% black or African American, 1.0% Asian, 0.2% American Indian, 0.7% from other races, and 2.5% from two or more races. Those of Hispanic or Latino origin made up 1.9% of the population. In terms of ancestry, 21.7% were German, 17.0% were American, 12.9% were Irish, and 10.8% were English. Of the 45,855 households, 29.5% had children under the age of 18 living with them, 45.7% were married couplesliving together, 14.1% had a female householder with no husband present, 36.0% were non-families, and 30.9% of all households were made up of individuals. The average household size was 2.33 and the average family size was 2.89. The median age was 40.3 years. The median income for a household in the county was $44,337 and the median income for a family was $57,570. Males had a median income of $48,570 versus $31,568 for females. The per capita income for the county was $24,726. About 10.3% of families and 15.7% of the population were below the poverty line, including 26.1% of those under age 18 and 6.5% of those age 65 or over.
In its early years Macon County favored the Democratic Party, voting for it in every election through 1856. Republican Abraham Lincoln won the county in his landmark 1860 election, and from then until the Great Depression Macon County became solidly Republican, only giving a narrow plurality to Woodrow Wilson in 1912 when the GOP was mortally divided by Theodore Roosevelt's splinter–party run. The FDR-era New Deal saw the county turn Democratic again due to its strong industrial base, although Macon was a perfect bellwether between 1932 and 1996 apart from the Catholicism-influenced 1960 election and the 1988 election heavily influenced by a major Midwestern drought. Al Gore did narrowly hold the county in 2000 despite losing the election due to a razor-thin loss in Florida, but since then the county has trended Republican. Illinois-bred Barack Obama did win Macon County in his 2008 triumph, but was convincingly defeated by Mitt Romney in 2012, whilst in 2016 Hillary Clinton suffered the worst Democratic loss since George McGovern.
