St. Marys, PA has a population of 12,748 and is the 3,259th largest city in the United States. The population density is 128 per sq mi which is 54% lower than the Pennsylvania average and 42% higher than the national average. The median age in St. Marys is 48 which is approximately 17% higher than the Pennsylvania average of 41. In St. Marys, 64% of the population over 15 years of age are married, 98% speak English and 0% speak Spanish. 90% of St. Marys residents were born in Pennsylvania, 8% were born out of state, 0% were born outside of the United States and 2% were foreign born.