Seneca, IL has a population of 2,565 and is the 9,689th largest city in the United States. The population density is 391 per sq mi which is 76% higher than the Illinois average and 331% higher than the national average. The median age in Seneca is 39 which is approximately 4% higher than the Illinois average of 37. In Seneca, 50% of the population over 15 years of age are married, 99% speak English and 1% speak Spanish. 84% of Seneca residents were born in Illinois, 16% were born out of state, 0% were born outside of the United States and 0% were foreign born.