Johnston, IA has a population of 19,491 and is the 2,234th largest city in the United States. The population density is 1,159 per sq mi which is 2022% higher than the Iowa average and 1179% higher than the national average. The median age in Johnston is 39 which is approximately 3% higher than the Iowa average of 38. In Johnston, 64% of the population over 15 years of age are married, 89% speak English and 1% speak Spanish. 65% of Johnston residents were born in Iowa, 25% were born out of state, 1% were born outside of the United States and 9% were foreign born.