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. Johnston is the location of the headquarters of Pioneer Hi-Bred Seeds, Iowa Public Television, and The Gardeners of America/Men's Garden Clubs of America.