Monmouth, IL has a population of 9,272 and is the 4,161st largest city in the United States. The population density is 2,193 per sq mi which is 886% higher than the Illinois average and 2320% higher than the national average. The median age in Monmouth is 35 which is approximately 8% lower than the Illinois average of 37. In Monmouth, 51% of the population over 15 years of age are married, 83% speak English and 13% speak Spanish. 73% of Monmouth residents were born in Illinois, 17% were born out of state, 0% were born outside of the United States and 10% were foreign born.