Fairfield, IL has a population of 4,944 and is the 6,533rd largest city in the United States. The population density is 1,224 per sq mi which is 451% higher than the Illinois average and 1251% higher than the national average. The median age in Fairfield is 43 which is approximately 15% higher than the Illinois average of 37. In Fairfield, 63% of the population over 15 years of age are married, 96% speak English and 2% speak Spanish. 77% of Fairfield residents were born in Illinois, 21% were born out of state, 0% were born outside of the United States and 2% were foreign born.