| | Marathon, FL | Key Colony Beach, FL | United States |
| Population | 9,622 | 744 | 329,725,481 |
| Female Population | 47.8% | 54.4% | 50.5% |
| Male Population | 52.2% | 45.6% | 49.5% |
| Median Age | 48.9 | 66.9 | 38.4 |
| space3 | | | |
| Population - 2010 | 8,319 | 796 | 309,327,143 |
| Population - 2000 | 10,255 | 788 | 285,036,114 |
| Population - 1990 | 8,857 | 977 | 251,960,433 |
| Pop. 2000 to Now | -12.4% | 0.9% | 15.6% |
| Pop. 1990 to Now | 1.4% | -18.6% | 30.8% |
| space4 | | | |
| Population Density | 1,139.2 | 1,690.9 | 93.4 |
| Land Area | 8.4 | 0.4 | 3,531,905.4 |
| Water Area | 0.8 | 0.2 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Marathon, FL | Key Colony Beach, FL | United States |
| Age 0 to 4 | 4.7% | 0.0% | 5.9% |
| Age 5 to 9 | 2.5% | 0.0% | 6.1% |
| Age 10 to 14 | 6.6% | 1.2% | 6.6% |
| Age 15 to 17 | 2.7% | 1.2% | 3.9% |
| Age 18 to 20 | 3.3% | 0.0% | 4.0% |
| Age 21 to 24 | 3.6% | 0.0% | 5.2% |
| Age 25 to 34 | 9.2% | 3.1% | 13.8% |
| Age 35 to 44 | 12.2% | 2.4% | 12.9% |
| Age 45 to 54 | 15.5% | 15.7% | 12.6% |
| Age 55 to 59 | 8.9% | 8.7% | 6.7% |
| Age 60 to 64 | 7.9% | 10.6% | 6.3% |
| Age 65 to 74 | 14.6% | 29.2% | 9.6% |
| Age 75 to 84 | 7.1% | 18.1% | 4.5% |
| Age 85 and over | 1.1% | 9.7% | 1.9% |
| space1 | | | |
| Race | | | |
| | Marathon, FL | Key Colony Beach, FL | United States |
| White | 54.4% | 93.8% | 59.4% |
| Black | 4.5% | 0.0% | 12.2% |
| Asian | 0.5% | 0.0% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.0% | 0.4% |
| Two or More Races | 1.0% | 0.0% | 3.2% |
| Hispanic | 39.5% | 6.2% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Marathon, FL | Key Colony Beach, FL | United States |
| Married Population | 49.2% | 59.3% | 49.8% |
| Currently Married | 47.0% | 59.3% | 48.0% |
| Married but Separated | 2.2% | 0.0% | 1.8% |
| Single Population | 50.8% | 40.7% | 50.2% |
| Never Married | 24.8% | 10.5% | 33.8% |
| Divorced | 18.6% | 16.7% | 10.8% |
| Widowed | 7.5% | 13.5% | 5.6% |
| space5 | | | |
| Household Size | 2.40 | 1.81 | 2.60 |
| Households | 3,996 | 410 | 124,010,992 |
| Family Households | 2,235 | 228 | 80,755,759 |
| Married couple, w/children | 21.4% | 3.5% | 30.5% |
| Married couple, no children | 58.0% | 90.8% | 42.9% |
| Non Family Households | 1,761 | 182 | 43,255,233 |
| Single householder, w/children | 11.5% | 0.0% | 15.8% |
| Single householder, no children | 9.1% | 5.7% | 10.8% |