was formed on November 2, 1829, and named in recognition of the citizens' petition for a new county, which said that they were petitioning "in the spirit of Union and Unity." The county is directly adjacent to the south to Union Parish in the state of Louisiana. The El Dorado, AR Micropolitan Statistical Area includes all of Union County.

Called by boosters the "Queen City of South Arkansas", El Dorado was at the heart of the 1920s