| 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 |
| Mercer County | 120 293 | 121 003 | - 710 | - 0.6 |
| Clark borough | 633 | 610 | 23 | 3.8 |
| Coolspring township | 2 287 | 2 140 | 147 | 6.9 |
| Deer Creek township | 465 | 513 | - 48 | - 9.4 |
| Delaware township | 2 159 | 2 064 | 95 | 4.6 |
| East Lackawannock township | 1 701 | 1 606 | 95 | 5.9 |
| Fairview township | 1 036 | 910 | 126 | 13.8 |
| Farrell city | 6 050 | 6 841 | - 791 | - 11.6 |
| Findley township | 2 305 | 2 284 | 21 | 0.9 |
| Fredonia borough | 652 | 683 | - 31 | - 4.5 |
| French Creek township | 764 | 789 | - 25 | - 3.2 |
| Greene township | 1 153 | 1 247 | - 94 | - 7.5 |
| Greenville borough | 6 380 | 6 734 | - 354 | - 5.3 |
| Grove City borough | 8 024 | 8 240 | - 216 | - 2.6 |
| Hempfield township | 4 004 | 3 826 | 178 | 4.7 |
| Hermitage city | 16 157 | 15 300 | 857 | 5.6 |
| Jackson township | 1 206 | 1 089 | 117 | 10.7 |
| Jackson Center borough | 221 | 244 | - 23 | - 9.4 |
| Jamestown borough | 636 | 761 | - 125 | - 16.4 |
| Jefferson township | 2 416 | 1 998 | 418 | 20.9 |
| Lackawannock township | 2 561 | 2 677 | - 116 | - 4.3 |
| Lake township | 706 | 651 | 55 | 8.4 |
| Liberty township | 1 276 | 1 223 | 53 | 4.3 |
| Mercer borough | 2 391 | 2 444 | - 53 | - 2.2 |
| Mill Creek township | 639 | 604 | 35 | 5.8 |
| New Lebanon borough | 205 | 209 | - 4 | - 1.9 |
| New Vernon township | 524 | 493 | 31 | 6.3 |
| Otter Creek township | 611 | 583 | 28 | 4.8 |
| Perry township | 1 471 | 1 468 | 3 | 0.2 |
| Pine township | 4 493 | 4 193 | 300 | 7.2 |
| Pymatuning township | 3 782 | 3 736 | 46 | 1.2 |
| Salem township | 769 | 678 | 91 | 13.4 |
| Sandy Creek township | 848 | 806 | 42 | 5.2 |
| Sandy Lake borough | 743 | 722 | 21 | 2.9 |
| Sandy Lake township | 1 248 | 1 161 | 87 | 7.5 |
| Sharon city | 16 328 | 17 493 | - 1 165 | - 6.7 |
| Sharpsville borough | 4 500 | 4 729 | - 229 | - 4.8 |
| Sheakleyville borough | 164 | 145 | 19 | 13.1 |
| Shenango township | 4 037 | 4 339 | - 302 | - 7.0 |
| South Pymatuning township | 2 857 | 2 775 | 82 | 3.0 |
| Springfield township | 1 972 | 1 892 | 80 | 4.2 |
| Stoneboro borough | 1 104 | 1 091 | 13 | 1.2 |
| Sugar Grove township | 909 | 987 | - 78 | - 7.9 |
| West Middlesex borough | 929 | 982 | - 53 | - 5.4 |
| West Salem township | 3 565 | 3 547 | 18 | 0.5 |
| Wheatland borough | 748 | 760 | - 12 | - 1.6 |
| Wilmington township | 1 105 | 1 177 | - 72 | - 6.1 |
| Wolf Creek township | 729 | 653 | 76 | 11.6 |
| Worth township | 830 | 906 | - 76 | - 8.4 |
| Source: U.S. Census Bureau | Prepared by: Pennsylvania State Data Center | |||