Johnston is a city in Polk County, Iowa, United States. The population was 24,064 in the 2020 census, a large increase from the 8,649 populatio…Johnston is a city in Polk County, Iowa, United States. The population was 24,064 in the 2020 census, a large increase from the 8,649 population in 2000. It is part of the Des Moines–West Des Moines Metropolitan Statistical Area.