Marion, IL has a population of 17,703 and is the 2,443rd largest city in the United States. The population density is 1,075 per sq mi which is 384% higher than the Illinois average and 1087% higher than the national average. The median age in Marion is 40 which is approximately 7% higher than the Illinois average of 37. In Marion, 59% of the population over 15 years of age are married, 96% speak English and 2% speak Spanish. 75% of Marion residents were born in Illinois, 21% were born out of state, 1% were born outside of the United States and 3% were foreign born.