Mifflinburg was established by legislation as the first county seat until it was moved to New Berlin in 1815.

[4] Union County is one of the 423 counties served by the Appalachian Regional Commission,[5] and it is identified as part of the "Midlands" by Colin Woodard in his book American Nations: A History of the Eleven Rival Regional Cultures of North America.

The United States Office of Management and Budget[15] has designated Union County as the Lewisburg, PA Micropolitan Statistical Area (μSA).

[citation needed] Andrew Jackson was the last Democratic Party candidate to win the county, in 1832.

The county has been solidly Republican in Presidential elections since John C. Frémont's win against James Buchanan in 1856.

Robert P. Casey is the only Democratic gubernatorial candidate to win the county in the last fifty years.

[22] The county seat of Lewisburg is heavily Democratic, having given Joe Biden over 70% of its vote in 2020.

[23] Under Pennsylvania law, there are four types of incorporated municipalities: cities, boroughs, townships, and, in most cases, towns.

The following boroughs and townships are located in Union County: Census-designated places are geographical areas designated by the U.S. Census Bureau for the purposes of compiling demographic data.