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County · code 17131

Mercer CountyIllinois

Mercer County is in Illinois with its county seat in Aledo, formed on January 13, 1825.

  • Founded1825
  • County seatAledo
  • Named forHugh Mercer
  • Land area561 sq mi + 7.5 sq mi water
  • Population15,699 2020 Census
  • Parcels mapped13,979 checked 15 October 2026
Open the free map Click any parcel in Mercer County: acreage, parcel id, address and who publishes the map. No sign-in.

Updated

Rock IslandWarrenHendersonMuscatine, IALouisa, IADes Moines, IAHenry Aledo Census outline · seat marked

Where it is

Mercer on the map of Illinois

Illinois has 102 counties and county equivalents, each with its own records office. Shapes with a page are links.

Adams CountyAlexander CountyBond CountyBoone CountyBrown CountyBureau CountyCalhoun CountyCarroll CountyCass CountyChampaign CountyChristian CountyClark CountyClay CountyClinton CountyColes CountyCook CountyCrawford CountyCumberland CountyDeKalb CountyDe Witt CountyDouglas CountyDuPage CountyEdgar CountyEdwards CountyEffingham CountyFayette CountyFord CountyFranklin CountyFulton CountyGallatin CountyGreene CountyGrundy CountyHamilton CountyHancock CountyHardin CountyHenderson CountyHenry CountyIroquois CountyJackson CountyJasper CountyJefferson CountyJersey CountyJo Daviess CountyJohnson CountyKane CountyKankakee CountyKendall CountyKnox CountyLake CountyLaSalle CountyLawrence CountyLee CountyLivingston CountyLogan CountyMcDonough CountyMcHenry CountyMcLean CountyMacon CountyMacoupin CountyMadison CountyMarion CountyMarshall CountyMason CountyMassac CountyMenard CountyMonroe CountyMontgomery CountyMorgan CountyMoultrie CountyOgle CountyPeoria CountyPerry CountyPiatt CountyPike CountyPope CountyPulaski CountyPutnam CountyRandolph CountyRichland CountyRock Island CountySt. Clair CountySaline CountySangamon CountySchuyler CountyScott CountyShelby CountyStark CountyStephenson CountyTazewell CountyUnion CountyVermilion CountyWabash CountyWarren CountyWashington CountyWayne CountyWhite CountyWhiteside CountyWill CountyWilliamson CountyWinnebago CountyWoodford CountyMercer County Mercer
Counties of Illinois · Mercer highlighted

History

Formed in 1825 from Pike County and Schuyler County

An old deed may name a county that no longer exists, or a boundary that has since moved.

Mercer County was created from Pike County, initially divided into two parts and attached to Schuyler County for county purposes. The county was named after Hugh Mercer and became fully organized and detached from Warren County in 1835. In 1837, Mercer County boundaries were extended to the middle of the Mississippi River.

Boundary changes from the Newberry Library, Atlas of Historical County Boundaries.

  1. 1825
    Mercer County created Mercer created from Non-County Area 3 attached to Pike; Mercer was divided into two parts. Mercer not fully organized, attached to Schuyler "for all county purposes."
    from Pikefrom Schuyler
    · Ill. Laws 1824-1825, pp. 85-87, 92-96
  2. 1826
    Changed Mercer detached from Schuyler, attached to Peoria "for county purposes."
    SchuylerPeoria
    · Ill. Laws 1826, pp. 76-77
  3. 1827
    Lost territory Mercer lost to creation of Jo Daviess.
    to Jo Daviess
    · Ill. Rev. Code 1826-1827, pp. 117-118
  4. 1831
    Changed Mercer detached from Peoria, attached to Warren "for judicial and other purposes."
    PeoriaWarren
    · Ill. Laws 1830-1831, pp. 67-68
  5. 1835
    Changed Mercer fully organized, detached from Warren.
    Warren
    · Ill. Laws 1835-1835, p. 156
  6. 1837
    Changed Mercer boundaries were extended to the middle of the Mississippi River. · Ill. Laws 1836-1837, pp. 91-92
  7. 1849
    Changed Legislature authorized Rock Island to gain from Mercer township 15 north, ranges 1 and 2 west, dependent on local referendum that failed (no change).
    Rock Island
    · Ill. Laws 1849, 1st sess., gen., pp. 51-52

Land and parcels

What the maps cover in Mercer

Who publishes each layer, how good it is, and when we last checked.

Official
from the office that keeps the record
Good enough
a reliable statewide or national copy
Fallback
a guide only; check the official record
Missing
nothing public found yet

People and land

Mercer by the Census

Population
15,699 2020 Census count
Housing units
7,291 2020 Census count
Land area
561 square miles
Water area
7.5 square miles
People per sq mi
28 population ÷ land area

Source: US Census Bureau, 2020 Census; US Census Bureau, 2020 Gazetteer Files. Not endorsed or certified by the Census Bureau.

Rules and offices

Who to ask before you build, dig or sell

Official county pages only. We link to them; we do not copy them, because they change.

Inside a city or town, permits usually come from that city rather than the county.

Permits

Zoning and code

County government

See your own lot on these maps

Open Mercer County on Parcel Chart's free map and click your parcel. Save it with a free account; the full record of your place (site map, projects, documents) is Pro, in early access.

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Questions

Common questions about Mercer County

When was Mercer County founded?
Mercer County was founded in 1825, formed from Pike County and Schuyler County, and named after Hugh Mercer.
What is the county seat of Mercer County?
The county seat of Mercer County is Aledo.
Is there an online parcel map for Mercer County?
Yes. A public source publishes a parcel map of 13,979 parcels, rated fallback here. You can click any parcel on it in Parcel Chart's free map, no sign-in needed.
Has Mercer County's boundary or name changed?
Mercer County was created in 1825 from Pike County and Schuyler County. Since then the Atlas of Historical County Boundaries records 5 later changes, chiefly involving Peoria County, Warren County and Jo Daviess County.
Who issues building permits in Mercer County?
The county's page on building permits: BUILDING PERMITS. Inside a city or town, ask that city first.
Is my lot in a flood zone?
We do not have flood maps for Mercer County yet. Look up the address on FEMA's Flood Map Service Center, which shows the effective flood map for any lot.

Nearby

Counties next door