Lanark, IL has a population of 1,542 and is the 12,516th largest city in the United States. The population density is 1,371 per sq mi which is 501% higher than the Illinois average and 1413% higher than the national average. The median age in Lanark is 43 which is approximately 15% higher than the Illinois average of 37. In Lanark, 63% of the population over 15 years of age are married, 96% speak English and 3% speak Spanish. 78% of Lanark residents were born in Illinois, 20% were born out of state, 0% were born outside of the United States and 2% were foreign born.