Marengo, IL has a population of 8,081 and is the 4,648th largest city in the United States. The population density is 1,022 per sq mi which is 360% higher than the Illinois average and 1028% higher than the national average. The median age in Marengo is 39 which is approximately 3% higher than the Illinois average of 37. In Marengo, 63% of the population over 15 years of age are married, 85% speak English and 12% speak Spanish. 76% of Marengo residents were born in Illinois, 14% were born out of state, 1% were born outside of the United States and 9% were foreign born.