Union County was formed out of Johnson County, nearly a year before the Illinois Territory gained statehood. It was named for a joint revival meeting of the Baptists and Dunkards, called a "union meeting". The county seal depicts the leaders of these two groups shaking hands.
For its first year of existence, Union County included an area of unorganized territory temporarily attached to it.
In 1819, the additional territory became Alexander County, reducing Union to its current borders.
According to the US Census Bureau, the county has a total area of 422 square miles (1,090 km2), of which 413 square miles (1,070 km2) is land and 8.7 square miles (23 km2) (2.1%) is water.
In recent years, average temperatures in the county seat of Jonesboro have ranged from a low of 22 °F (−6 °C) in January to a high of 90 °F (32 °C) in July, although a record low of −20 °F (−29 °C) was recorded in January 1918 and a record high of 112 °F (44 °C) was recorded in July 1901. Average monthly precipitation ranged from 3.13 inches (80 mm) in September to 5.22 inches (133 mm) in May.
As of the 2010 United States Census, there were 17,808 people, 7,167 households, and 4,837 families residing in the county. The population density was 43.1 inhabitants per square mile (16.6/km2). There were 7,924 housing units at an average density of 19.2 per square mile (7.4/km2). The racial makeup of the county was 94.8% white, 0.9% black or African American, 0.5% American Indian, 0.3% Asian, 2.0% from other races, and 1.5% from two or more races. Those of Hispanic or Latino origin made up 4.8% of the population. In terms of ancestry, 23.0% were German, 12.6% were Irish, 9.4% were English, and 8.1% were American.
Of the 7,167 households, 29.7% had children under the age of 18 living with them, 52.6% were married couples living together, 10.6% had a female householder with no husband present, 32.5% were non-families, and 28.4% of all households were made up of individuals. The average household size was 2.41 and the average family size was 2.92. The median age was 42.9 years.
The median income for a household in the county was $39,760 and the median income for a family was $48,465. Males had a median income of $36,831 versus $31,272 for females. The per capita income for the county was $19,512. About 12.7% of families and 21.1% of the population were below the poverty line, including 22.5% of those under age 18 and 16.7% of those age 65 or over.
^Callary, Edward (2009). Place Names of Illinois. Urbana and Chicago, Illinois: University of Illinois Press. p. 354.
^White, Jesse. Origin and Evolution of Illinois Counties. State of Illinois, March 2010. "Archived copy"(PDF). Archived(PDF) from the original on October 12, 2012. Retrieved September 26, 2012.CS1 maint: archived copy as title (link)