Marathon, FL has a population of 8,701 and is the 4,383rd largest city in the United States. The population density is 1,031 per sq mi which is 251% higher than the Florida average and 1038% higher than the national average. The median age in Marathon is 42 which is approximately equal to the Florida average of 42. In Marathon, 46% of the population over 15 years of age are married, 66% speak English and 30% speak Spanish. 27% of Marathon residents were born in Florida, 45% were born out of state, 1% were born outside of the United States and 27% were foreign born.