10 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Morgan City | 1 | 10,663 | 11,411 | -1.39% | 1,782 | 6 | City |
| Franklin | 2 | 6,187 | 6,670 | -1.54% | 621 | 10 | City |
| Patterson | 3 | 5,540 | 5,907 | -1.32% | 2,195 | 3 | City |
| Berwick | 4 | 4,432 | 4,744 | -1.4% | 751 | 6 | Town |
| Bayou Vista | 5 | 3,778 | 4,346 | -1.51% | 2,097 | 2 | CDP |
| Amelia | 6 | 1,885 | 2,236 | -2.03% | 748 | 3 | CDP |
| Baldwin | 7 | 1,644 | 1,747 | -1.26% | 511 | 3 | Town |
| Charenton | 8 | 838 | 1,560 | -7.81% | 169 | 5 | CDP |
| Centerville | 9 | 776 | 331 | 7.18% | 824 | 1 | CDP |
| Glencoe | 10 | 196 | 258 | -0.51% | 296 | 1 | CDP |
Morgan City is the largest city in St. Mary Parish, with a population of 10,663.
Glencoe is the smallest city in St. Mary Parish, with a population of 196.
There are 10 cities in St. Mary Parish, Louisiana.