Illinois · FIPS 17131 · County seat: Aledo
Mercer County, IL GIS & Parcel Viewer
Here is the direct link to the county parcel map covering Mercer County, IL: Mercer County Portico map (Sidwell). Below you'll find basic facts about the county, steps for using the viewer, and nearby counties.
Open the Mercer County parcel map ↗
Opens portico.mygisonline.com in a new tab. We link out and never embed or copy parcel data.
Map and county facts
| Official map | Mercer County Portico map (Sidwell) |
|---|---|
| Type | County-run map |
| Website | portico.mygisonline.com |
| Access | Free, no login (checked Oct 2026) |
| County seat | Aledo |
| Population | 15,487 (2023 Census Bureau estimate) |
| Land area | 561 sq mi |
| FIPS code | 17131 |
Where the parcel data comes from
Mercer County runs this viewer (directly or through a contracted vendor), which makes it the closest thing to the source for local parcel lines. As of our October 2026 test, anyone could open it without signing in or paying.
Finding a property on this map
- Press the button above. This county's viewer runs on myGISonline; 13 counties in our directory use the same service.
- Find the search option and look up a property, or zoom in on the map.
- Click a lot to see its details.
- Check the toolbar for layers, measuring and printing.
Many parcel sites expect owner names last name first. If a full name finds nothing, try just the surname.
The county in brief
The U.S. Census Bureau estimated Mercer County's population at 15,487 in 2023, spread over about 561 square miles of land, or roughly 28 people per square mile. By population it ranks 69th of 102 counties in Illinois. Aledo serves as the county seat.
Neighboring counties
If the property sits near a county line, the parcel may belong to a neighbor. These counties border Mercer County:
- Henderson County
- Henry County
- Knox County
- Rock Island County
- Warren County
- Des Moines County, IA
- Louisa County, IA
Before you rely on the lines
The lines you see online are approximate. They're good for finding a lot and its parcel number, but a legal boundary question needs the recorded plat or a surveyor.