Alexander County is the southernmost county of the U.S. state of Illinois. As of the 2010 census, the population was 8,238. Its county seat is Cairo and its western boundary is formed by the Mississippi River.
According to the U.S. Census Bureau, the county has a total area of 253 square miles (660 km), of which 236 square miles (610 km) is land and 17 square miles (44 km) (6.8%) is water. Its borders are partly defined by the Mississippi River and the Ohio River. The lowest point in the state of Illinois is located on the Mississippi River in Cairo in Alexander County, where it flows out of Illinois and into Kentucky.
As of the 2010 United States Census, there were 8,238 people, 3,329 households, and 2,093 families residing in the county. The population density was 35.0 inhabitants per square mile (13.5/km). There were 4,006 housing units at an average density of 17.0 per square mile (6.6/km). The racial makeup of the county was 60.9% white, 35.4% black or African American, 0.3% American Indian, 0.2% Asian, 0.1% Pacific islander, 1.4% from other races, and 1.7% from two or more races. Those of Hispanic or Latino origin made up 1.9% of the population. In terms of ancestry, 13.9% were German, 6.8% were Irish, 5.3% were English, and 4.7% were American.
Of the 3,329 households, 29.6% had children under the age of 18 living with them, 39.6% were married couples living together, 18.5% had a female householder with no husband present, 37.1% were non-families, and 33.6% of all households were made up of individuals. The average household size was 2.31 and the average family size was 2.94. The median age was 41.1 years.
The median income for a household in the county was $28,833 and the median income for a family was $44,699. Males had a median income of $35,880 versus $25,743 for females. The per capita income for the county was $15,858. About 11.8% of families and 20.1% of the population were below the poverty line, including 33.1% of those under age 18 and 14.7% of those age 65 or over.