| | Marathon, FL | Key West, FL | United States |
| Population | 9,622 | 26,776 | 329,725,481 |
| Female Population | 47.8% | 47.0% | 50.5% |
| Male Population | 52.2% | 53.0% | 49.5% |
| Median Age | 48.9 | 44.2 | 38.4 |
| space3 | | | |
| Population - 2010 | 8,319 | 24,662 | 309,327,143 |
| Population - 2000 | 10,255 | 25,478 | 285,036,114 |
| Population - 1990 | 8,857 | 24,832 | 251,960,433 |
| Pop. 2000 to Now | -12.4% | -6.7% | 15.6% |
| Pop. 1990 to Now | 1.4% | -4.3% | 30.8% |
| space4 | | | |
| Population Density | 1,139.2 | 4,790.0 | 93.4 |
| Land Area | 8.4 | 5.6 | 3,531,905.4 |
| Water Area | 0.8 | 1.6 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Marathon, FL | Key West, FL | United States |
| Age 0 to 4 | 4.7% | 4.0% | 5.9% |
| Age 5 to 9 | 2.5% | 3.5% | 6.1% |
| Age 10 to 14 | 6.6% | 4.6% | 6.6% |
| Age 15 to 17 | 2.7% | 2.3% | 3.9% |
| Age 18 to 20 | 3.3% | 2.5% | 4.0% |
| Age 21 to 24 | 3.6% | 3.4% | 5.2% |
| Age 25 to 34 | 9.2% | 15.5% | 13.8% |
| Age 35 to 44 | 12.2% | 15.4% | 12.9% |
| Age 45 to 54 | 15.5% | 13.3% | 12.6% |
| Age 55 to 59 | 8.9% | 9.4% | 6.7% |
| Age 60 to 64 | 7.9% | 7.3% | 6.3% |
| Age 65 to 74 | 14.6% | 12.0% | 9.6% |
| Age 75 to 84 | 7.1% | 4.8% | 4.5% |
| Age 85 and over | 1.1% | 2.0% | 1.9% |
| space1 | | | |
| Race | | | |
| | Marathon, FL | Key West, FL | United States |
| White | 54.4% | 58.9% | 59.4% |
| Black | 4.5% | 12.3% | 12.2% |
| Asian | 0.5% | 1.7% | 5.6% |
| Native American | 0.0% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.2% | 0.4% |
| Two or More Races | 1.0% | 2.1% | 3.2% |
| Hispanic | 39.5% | 24.7% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Marathon, FL | Key West, FL | United States |
| Married Population | 49.2% | 45.1% | 49.8% |
| Currently Married | 47.0% | 43.0% | 48.0% |
| Married but Separated | 2.2% | 2.1% | 1.8% |
| Single Population | 50.8% | 54.9% | 50.2% |
| Never Married | 24.8% | 34.1% | 33.8% |
| Divorced | 18.6% | 16.0% | 10.8% |
| Widowed | 7.5% | 4.8% | 5.6% |
| space5 | | | |
| Household Size | 2.40 | 2.32 | 2.60 |
| Households | 3,996 | 11,002 | 124,010,992 |
| Family Households | 2,235 | 5,748 | 80,755,759 |
| Married couple, w/children | 21.4% | 23.7% | 30.5% |
| Married couple, no children | 58.0% | 55.2% | 42.9% |
| Non Family Households | 1,761 | 5,254 | 43,255,233 |
| Single householder, w/children | 11.5% | 14.3% | 15.8% |
| Single householder, no children | 9.1% | 6.8% | 10.8% |