Mason City is a city in Mason County, Illinois, United States. The population was 2,077 at the 2020 census, down from 2,343 in 2010. …Mason City is a city in Mason County, Illinois, United States. The population was 2,077 at the 2020 census, down from 2,343 in 2010. Mason City was established in 1857. The city took its name from Mason County.