According to the U.S. Census Bureau, the county has a total area of, of which is land and is water. The county has the unusual distinction of forming a perfect square by Mercator map projection. The county seat, Macomb, is in the center of the county, and the courthouse is almost precisely in the center of the county.
As of the 2010 United States Census, there were 32,612 people, 13,057 households, and 6,724 families residing in the county. The population density was. There were 14,419 housing units at an average density of. The racial makeup of the county was 90.4% white, 5.0% black or African American, 1.8% Asian, 0.2% American Indian, 0.7% from other races, and 1.8% from two or more races. Those of Hispanic or Latino origin made up 2.7% of the population. In terms of ancestry, 25.0% were German, 14.7% were American, 13.7% were Irish, 12.7% were English, and 5.1% were Italian. Of the 13,057 households, 22.0% had children under the age of 18 living with them, 40.0% were married couples living together, 8.0% had a female householder with no husband present, 48.5% were non-families, and 33.9% of all households were made up of individuals. The average household size was 2.19 and the average family size was 2.80. The median age was 28.9 years. The median income for a household in the county was $33,702 and the median income for a family was $52,390. Males had a median income of $42,297 versus $28,530 for females. The per capita income for the county was $18,344. About 12.8% of families and 23.7% of the population were below the poverty line, including 21.7% of those under age 18 and 9.4% of those age 65 or over.
Politics
McDonough County is located in Illinois’ 18th Congressional District and is currently represented by Republican Darin LaHood. For the Illinois House of Representatives, the county is located in the 93rd district and is currently represented by Republican Norine Hammond. The county is located in the 47th district of the Illinois Senate, and is currently represented by Republican Jil Tracy. In presidential elections, McDonough County was once reliably Republican, voting for the Republican candidate in all but two elections from 1892 to 1988. Since 1988, the county has become more competitive, with Democrats carrying the county three out of seven times and holding the Republican candidate's margin of victory to under four points three times.
Climate and weather
In recent years, average temperatures in the county seat of Macomb 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 August 1934. Average monthly precipitation ranged from in January to in May.
