Union is a home-rule-class city in Boone County, Kentucky, United States. The population was 7,416 as of the 2020 United States Census. The …Union is a home-rule-class city in Boone County, Kentucky, United States. The population was 7,416 as of the 2020 United States Census. The area was rural until residential growth in the 1990s and 2000s. Union is located 17 miles southwest of Cincinnati, Ohio.