| Population Change 1990-2000 | ||||
| County or Municipality | 2000 Population | 1990 Population | Population Change | Percent Change |
| Pennsylvania | 12 281 054 | 11 881 643 | 399 411 | 3.4 |
| Mifflin County | 46 486 | 46 197 | 289 | 0.6 |
| Armagh township | 3 988 | 3 627 | 361 | 10.0 |
| Bratton township | 1 259 | 1 427 | - 168 | - 11.8 |
| Brown township | 3 852 | 3 320 | 532 | 16.0 |
| Burnham borough | 2 144 | 2 197 | - 53 | - 2.4 |
| Decatur township | 3 021 | 2 735 | 286 | 10.5 |
| Derry township | 7 256 | 7 650 | - 394 | - 5.2 |
| Granville township | 4 895 | 5 090 | - 195 | - 3.8 |
| Juniata Terrace borough | 502 | 556 | - 54 | - 9.7 |
| Kistler borough | 344 | 314 | 30 | 9.6 |
| Lewistown borough | 8 998 | 9 341 | - 343 | - 3.7 |
| McVeytown borough | 405 | 408 | - 3 | - 0.7 |
| Menno township | 1 763 | 1 637 | 126 | 7.7 |
| Newton Hamilton borough | 272 | 287 | - 15 | - 5.2 |
| Oliver township | 2 060 | 1 822 | 238 | 13.1 |
| Union township | 3 313 | 3 265 | 48 | 1.5 |
| Wayne township | 2 414 | 2 521 | - 107 | - 4.2 |
| Source: U.S. Census Bureau | Prepared by: Pennsylvania State Data Center | |||