Originally known as St. Marys Landing Name shortened to St. Marys after 1853. Name shortened again to simply St. Mary sometime after 1990.
| Year | Population | Percent change |
|---|---|---|
| 1900 | 576 | -- |
| 1910 | 702 | +21.9% |
| 1920 | 722 | +2.8% |
| 1930 | 554 | -23.3% |
| 1940 | 605 | +9.2% |
| 1950 | 635 | +5.0% |
| 1960 | 620 | -2.4% |
| 1970 | 645 | +4.0% |
| 1980 | 565 | -12.4% |
| 1990 | 461 | -18.4% |
| 2000 | 377 | -18.2% |