Union is a city in Hardin County, Iowa, United States. The population was 399 at the time of the 2020 census. Union was laid out in 1868 and w…Union is a city in Hardin County, Iowa, United States. The population was 399 at the time of the 2020 census. Union was laid out in 1868 and was incorporated as a town in 1874. Union is located at 42°14′41″N 93°3′51″W / 42.24472°N 93.06417°W.