St. Marys, GA has a population of 17,853 and is the 2,425th largest city in the United States. The population density is 790 per sq mi which is 373% higher than the Georgia average and 772% higher than the national average. The median age in St. Marys is 30 which is approximately 16% lower than the Georgia average of 36. In St. Marys, 60% of the population over 15 years of age are married, 95% speak English and 4% speak Spanish. 31% of St. Marys residents were born in Georgia, 66% were born out of state, 2% were born outside of the United States and 2% were foreign born.