Fairmont City, IL has a population of 2,349 and is the 10,157th largest city in the United States. The population density is 392 per sq mi which is 76% higher than the Illinois average and 332% higher than the national average. The median age in Fairmont City is 31 which is approximately 18% lower than the Illinois average of 37. In Fairmont City, 47% of the population over 15 years of age are married, 34% speak English and 66% speak Spanish. 44% of Fairmont City residents were born in Illinois, 20% were born out of state, 2% were born outside of the United States and 35% were foreign born.