[2] Since 2018, Union County has been included in the Cincinnati-Middletown, OH-KY-IN Metropolitan Statistical Area.

Prior to cession to the United States government, this area belonged to and was inhabited by peoples of the Chippewa, Delaware, Eel River, Kaskaskia, Kickapoo, Miami, Ottawa, Piankishaw, Potawatomi, Shawnee, Wea, and Wyandot tribes.

In 1790 the Territory was divided into two counties, with Knox covering much of present-day Indiana.

The first county seat was Brownsville, a small community on the East Fork of the Whitewater River.

Its low rolling hills have now been cleared of timber, although drainage areas are still largely brush-filled.

[7] The highest point on the terrain (1,140 feet/350 meters ASL) is a rise near the county's north border, 1.0 mile (1.6 km) ENE from Witts Station.

[8] Silver Creek flows southwestward through the northeastern part of the county, discharging into Whitewater Lake.

The East Fork of Whitewater River flows southward through the upper western part of the county, discharging into Brookville Lake.

The lower part of the county is drained by Dubois Creek, flowing westward into Brookville Lake.

The commissioners execute acts legislated by the council, collect revenue, and manage county government.

The judge of the court is elected to a term of six years and must be a member of the Indiana Bar Association.