Metropolis is a city and the county seat in Massac County, Illinois, United States. It is located by the Ohio River. As of the 2020 census, it had …Metropolis is a city and the county seat in Massac County, Illinois, United States. It is located by the Ohio River. As of the 2020 census, it had a population of 5,969, down from 6,537 in 2010. Metropolis is part of the Paducah, KY-IL Metropolitan Statistical Area in Southern Illinois.