Marathon is a city in the middle of the Florida Keys, in Monroe County, Florida, United States. As of the 2020 census, the city had a populati…Marathon is a city in the middle of the Florida Keys, in Monroe County, Florida, United States. As of the 2020 census, the city had a population of 9,689, up from 8,297 in 2010. Though the area has been settled for some time, Marathon is a relatively new city, incorporated in 1999.