Quincy, IL has a population of 40,689 and is the 1,014th largest city in the United States. The population density is 2,564 per sq mi which is 1053% higher than the Illinois average and 2729% higher than the national average. The median age in Quincy is 40 which is approximately 7% higher than the Illinois average of 37. In Quincy, 57% of the population over 15 years of age are married, 98% speak English and 1% speak Spanish. 76% of Quincy residents were born in Illinois, 22% were born out of state, 0% were born outside of the United States and 2% were foreign born.