Lake San Marcos, CA has a population of 4,529 and is the 6,898th largest city in the United States. The population density is 2,653 per sq mi which is 1042% higher than the California average and 2829% higher than the national average. The median age in Lake San Marcos is 60 which is approximately 66% higher than the California average of 36. In Lake San Marcos, 67% of the population over 15 years of age are married, 73% speak English and 17% speak Spanish. 39% of Lake San Marcos residents were born in California, 40% were born out of state, 0% were born outside of the United States and 21% were foreign born.