Mendon, IL has a population of 935 and is the 15,557th largest city in the United States. The population density is 1,088 per sq mi which is 390% higher than the Illinois average and 1101% higher than the national average. The median age in Mendon is 41 which is approximately 10% higher than the Illinois average of 37. In Mendon, 66% of the population over 15 years of age are married, 99% speak English and 1% speak Spanish. 87% of Mendon residents were born in Illinois, 13% were born out of state, 0% were born outside of the United States and 0% were foreign born.