| Statistic | St. Marks | Florida | National |
| Population | 340 | 21,477,737 | 328,239,523 |
| Population density (sq mi) | 174 | 410 | 91 |
| Median age | 68.1 | 41.6 | 37.7 |
| Male/Female ratio | 1.2:1 | 1.0:1 | 1.0:1 |
| Married (15yrs & older) | 80% | 54% | 55% |
| Families w/ Kids under 18 | 14% | 38% | 43% |
| Speak English | 99% | 72% | 79% |
| Speak Spanish | 1% | 21% | 13% |
| Race | St. Marks | Florida | National |
| White | 97.94% | 75.90% | 73.35% |
| Black | 0.00% | 16.14% | 12.63% |
| Asian | 0.00% | 2.61% | 5.22% |
| American Indian | 0.00% | 0.27% | 0.82% |
| Native Hawaiian | 0.00% | 0.06% | 0.18% |
| Mixed race | 2.06% | 2.48% | 3.06% |
| Other race | 0.00% | 2.54% | 4.75% |
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