St. Marys, IA has a population of 210 and is the 24,226th largest city in the United States. The population density is 1,479 per sq mi which is 2495% higher than the Iowa average and 1533% higher than the national average. The median age in St. Marys is 59 which is approximately 54% higher than the Iowa average of 38. In St. Marys, 72% of the population over 15 years of age are married, 100% speak English and 0% speak Spanish. 79% of St. Marys residents were born in Iowa, 21% were born out of state, 0% were born outside of the United States and 0% were foreign born.