Johnston is a city in Polk County, Iowa, United States. The population was 17,278 in the 2010 census, an increase of almost 100% from the 8,649 population in the 2000 census. 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. Also located here are the Camp Dodge Military Reservation as well as the Paul J.