Mercer, MO has a population of 411 and is the 20,725th largest city in the United States. The population density is 1,118 per sq mi which is 1194% higher than the Missouri average and 1134% higher than the national average. The median age in Mercer is 52 which is approximately 36% higher than the Missouri average of 38. In Mercer, 54% of the population over 15 years of age are married, 97% speak English and 0% speak Spanish. 46% of Mercer residents were born in Missouri, 53% were born out of state, 0% were born outside of the United States and 1% were foreign born.