St. Joseph, IL has a population of 4,109 and is the 7,329th largest city in the United States. The population density is 1,973 per sq mi which is 788% higher than the Illinois average and 2078% higher than the national average. The median age in St. Joseph is 34 which is approximately 9% lower than the Illinois average of 37. In St. Joseph, 64% of the population over 15 years of age are married, 99% speak English and 0% speak Spanish. 85% of St. Joseph residents were born in Illinois, 14% were born out of state, 0% were born outside of the United States and 1% were foreign born.