Mercer County, Illinois
County seat: Aledo · Population 15,229 (2025) · 561 sq mi
Mercer County is the 68th most populous of Illinois's 102 counties, with 15,229 people (2025 estimate). Its county seat is Aledo. It is part of the Northwest region, one of the 10 Illinois economic development regions (DCEO). The map shows its 12 cities and towns and the 7 neighboring counties.
Mercer County facts
- County seat
- Aledo
- Population (2025 estimate)
- 15,229 −3% since 2020
- Land area
- 561 sq mi 45th largest of 102
- Water area
- 8 sq mi
- Population density
- 27 per sq mi
- Established
- 1825
- Same name
- One of 8 Mercer Counties in the US
- Named after
- Hugh Mercer Jacobite and General in the American Revolutionary War
- Largest city
- Aledo 3,506
- Median household income
- $74,182
- Cities and towns
- 12 plus 6 CDPs
- ZIP codes
- 18
- Time zone
- Central
- FIPS code
- 17131
Cities and towns in Mercer County
Mercer County has 12 incorporated cities, towns and villages. Populations count the people living inside Mercer County; cities that cross a county line also list the other counties.
| Name | Type | Population 2025 | Also in |
|---|---|---|---|
| Aledo (county seat) | City | 3,506 | |
| Viola | Village | 836 | |
| Sherrard | Village | 691 | |
| Matherville | Village | 689 | |
| Windsor | Village | 649 | |
| New Boston | City | 600 | |
| Keithsburg | City | 546 | |
| Joy | Village | 357 | |
| Alexis | Village | 330 | Warren County |
| Seaton | Village | 208 | |
| North Henderson | Village | 159 | |
| Reynolds | Village | 37 | Rock Island County |
Unincorporated communities (CDPs)
Census-designated places are communities without their own city government; the county provides their services.
| Community | Population | Also in |
|---|---|---|
| Preemption | 127 | |
| Cable | 122 | |
| Swedona | 64 | |
| Millersburg | 33 | |
| Eliza | 0 | |
| Hamlet | 0 |
ZIP codes in Mercer County
18 ZIP codes reach into Mercer County. ZIP codes ignore county lines, so some also cover a neighboring county.
- 61231
- 61259 also Rock Island County
- 61260
- 61262 also Henry County
- 61263
- 61264 also Rock Island County
- 61272 also Rock Island County
- 61273 also Henry and Rock Island counties
- 61276
- 61279 also Rock Island County
- 61281 also Rock Island County
- 61412 also Warren County
- 61442 also Henderson County
- 61465 also Henry County
- 61466
- 61472 also Knox County
- 61476 also Henderson County
- 61486
Population of Mercer County, 2020–2025
Mercer County shrank by 472 people (−3%) between April 2020 and July 2025: 381 more deaths than births and a net loss of 85 people from migration.
| Year (July 1) | Population |
|---|---|
| 2020 | 15,637 |
| 2021 | 15,587 |
| 2022 | 15,511 |
| 2023 | 15,472 |
| 2024 | 15,292 |
| 2025 | 15,229 |
Neighboring counties
- Rock Island County northeast · 141,869 people
- Louisa County, Iowa west · 10,416 people
- Warren County south · 16,222 people
- Henry County east · 48,095 people
- Henderson County south · 5,984 people
- Knox County southeast · 47,767 people
- Des Moines County, Iowa southwest · 38,077 people
Printable Mercer County map
The PDF is a vector file with real text: it prints sharp on Letter paper and you can search it for a town. The PNG is 2400 pixels wide for slides and documents. Both are free to use with a credit to WhichCounty.
- PDF (Letter) (103 KB)
- PNG (2400 px) (72 KB)
Paste this code to embed the map on your site:
Sources
- Boundaries: U.S. Census Bureau, 2025 Cartographic Boundary Files (1:500,000).
- Population: Census Bureau county and city estimates, Vintage 2025. CDP populations: American Community Survey 2020–2024.
- Places and ZIP codes: Census Bureau 2026 Gazetteer and 2020 ZIP Code Tabulation Area relationship files.
- County seat, founding year and name origin: Wikidata (CC0), checked against state sources.
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