Johnston City, IL has a population of 3,521 and is the 8,021st largest city in the United States. The population density is 1,687 per sq mi which is 659% higher than the Illinois average and 1762% higher than the national average. The median age in Johnston City is 42 which is approximately 11% higher than the Illinois average of 37. In Johnston City, 54% of the population over 15 years of age are married, 98% speak English and 0% speak Spanish. 84% of Johnston City residents were born in Illinois, 13% were born out of state, 0% were born outside of the United States and 3% were foreign born.