| | Key Largo, FL | Marathon, FL | United States |
| Population | 10,854 | 9,622 | 329,725,481 |
| Female Population | 48.1% | 47.8% | 50.5% |
| Male Population | 51.9% | 52.2% | 49.5% |
| Median Age | 52 | 48.9 | 38.4 |
| space3 | | | |
| Population - 2010 | 11,540 | 8,319 | 309,327,143 |
| Population - 2000 | 11,886 | 10,255 | 285,036,114 |
| Population - 1990 | 11,336 | 8,857 | 251,960,433 |
| Pop. 2000 to Now | -18.4% | -12.4% | 15.6% |
| Pop. 1990 to Now | -14.5% | 1.4% | 30.8% |
| space4 | | | |
| Population Density | 900.7 | 1,139.2 | 93.4 |
| Land Area | 12.1 | 8.4 | 3,531,905.4 |
| Water Area | 3.2 | 0.8 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Key Largo, FL | Marathon, FL | United States |
| Age 0 to 4 | 3.5% | 4.7% | 5.9% |
| Age 5 to 9 | 3.9% | 2.5% | 6.1% |
| Age 10 to 14 | 5.7% | 6.6% | 6.6% |
| Age 15 to 17 | 1.4% | 2.7% | 3.9% |
| Age 18 to 20 | 1.3% | 3.3% | 4.0% |
| Age 21 to 24 | 2.8% | 3.6% | 5.2% |
| Age 25 to 34 | 8.6% | 9.2% | 13.8% |
| Age 35 to 44 | 13.0% | 12.2% | 12.9% |
| Age 45 to 54 | 15.8% | 15.5% | 12.6% |
| Age 55 to 59 | 8.2% | 8.9% | 6.7% |
| Age 60 to 64 | 8.1% | 7.9% | 6.3% |
| Age 65 to 74 | 17.1% | 14.6% | 9.6% |
| Age 75 to 84 | 6.8% | 7.1% | 4.5% |
| Age 85 and over | 3.6% | 1.1% | 1.9% |
| space1 | | | |
| Race | | | |
| | Key Largo, FL | Marathon, FL | United States |
| White | 70.7% | 54.4% | 59.4% |
| Black | 0.2% | 4.5% | 12.2% |
| Asian | 0.6% | 0.5% | 5.6% |
| Native American | 0.1% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.5% | 0.0% | 0.4% |
| Two or More Races | 0.9% | 1.0% | 3.2% |
| Hispanic | 27.0% | 39.5% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Key Largo, FL | Marathon, FL | United States |
| Married Population | 54.3% | 49.2% | 49.8% |
| Currently Married | 53.8% | 47.0% | 48.0% |
| Married but Separated | 0.4% | 2.2% | 1.8% |
| Single Population | 45.7% | 50.8% | 50.2% |
| Never Married | 21.8% | 24.8% | 33.8% |
| Divorced | 15.7% | 18.6% | 10.8% |
| Widowed | 8.3% | 7.5% | 5.6% |
| space5 | | | |
| Household Size | 2.35 | 2.40 | 2.60 |
| Households | 4,612 | 3,996 | 124,010,992 |
| Family Households | 2,846 | 2,235 | 80,755,759 |
| Married couple, w/children | 19.2% | 21.4% | 30.5% |
| Married couple, no children | 60.8% | 58.0% | 42.9% |
| Non Family Households | 1,766 | 1,761 | 43,255,233 |
| Single householder, w/children | 9.1% | 11.5% | 15.8% |
| Single householder, no children | 11.0% | 9.1% | 10.8% |