Fairmont, IL has a population of 2,516 and is the 9,807th largest city in the United States. The population density is 1,812 per sq mi which is 715% higher than the Illinois average and 1900% higher than the national average. The median age in Fairmont is 33 which is approximately 12% lower than the Illinois average of 37. In Fairmont, 35% of the population over 15 years of age are married, 78% speak English and 20% speak Spanish. 72% of Fairmont residents were born in Illinois, 15% were born out of state, 2% were born outside of the United States and 12% were foreign born.