Fairmount, IL has a population of 622 and is the 18,208th largest city in the United States. The population density is 1,959 per sq mi which is 781% higher than the Illinois average and 2063% higher than the national average. The median age in Fairmount is 41 which is approximately 10% higher than the Illinois average of 37. In Fairmount, 65% of the population over 15 years of age are married, 99% speak English and 1% speak Spanish. 84% of Fairmount residents were born in Illinois, 15% were born out of state, 0% were born outside of the United States and 0% were foreign born.