According to the U.S. Census Bureau, the county has a total area of, of which is land and is water. The Illinois River flows along part of the county's eastern border.
In recent years, average temperatures in the county seat of Mount Sterling 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 1936. Average monthly precipitation ranged from in January to in May.
Demographics
As of the 2010 United States Census, there were 6,937 people, 2,099 households, and 1,346 families living in the county. The population density was. There were 2,462 housing units at an average density of. The racial makeup of the county was 76.1% white, 18.5% black or African American, 0.2% Asian, 0.2% American Indian, 4.4% from other races, and 0.6% from two or more races. Those of Hispanic or Latino origin made up 5.8% of the population. In terms of ancestry, 25.5% were German, 15.6% were American, 12.7% were Irish, and 6.1% were English. Of the 2,099 households, 28.4% had children under the age of 18 living with them, 51.3% were married couples living together, 8.6% had a female householder with no husband present, 35.9% were non-families, and 32.2% of all households were made up of individuals. The average household size was 2.30 and the average family size was 2.88. The median age was 37.3 years. The median income for a household in the county was $38,696 and the median income for a family was $50,341. Males had a median income of $34,648 versus $27,288 for females. The per capita income for the county was $17,133. About 9.9% of families and 12.0% of the population were below the poverty line, including 15.0% of those under age 18 and 9.2% of those age 65 or over. The Illinois state prison, Western Illinois Correctional Center, located south east of Mount Sterling, has an average daily population of 2,066. The all-male population and racial makeup of the prison is so large, compared to the rest of the county, that it skews the census demographic data. This is evidenced by the lopsided age pyramid.
Brown County is located in Illinois's 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. Brown County is part of the German-settled area of western Central Illinois and the Missouri Rhineland centred on the St. Louis metropolitan area. This region was opposed both to southern plantation owners and socially liberal Yankees and consequently voted against the majority of people in their state. Brown County voted for the Democratic candidate in every presidential election from at least 1840 through 1948, with the sole exception of the 1928 election when the county voted for the Republican candidate Herbert Hoover. From 1952 to 2016, Democrats have carried Brown County only three times and have won a majority of the vote only once.
