Iowa City is the county seat and largest city of Johnson County, Iowa, United States. At the time of the 2020 census the population was 74,…Iowa City is the county seat and largest city of Johnson County, Iowa, United States. At the time of the 2020 census the population was 74,828, making it the state's fifth-most populous city. The Iowa City metropolitan area, which encompasses Johnson and Washington counties, has a population of over 171,000. The metro area is also a part of a combined statistical area with the Cedar Rapids metro area known as the Iowa City-Cedar Rapids region which collectively has a population of nearly 500,000.