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County · code 17199

Williamson CountyIllinois

Williamson County is in Illinois and has its county seat in Marion. The county was formed on August 5, 1839, from Franklin County and named after Hugh Williamson.

  • Founded1839
  • County seatMarion
  • Named forHugh Williamson
  • Land area420 sq mi + 24 sq mi water
  • Population67,153 2020 Census
Open the free map Click any parcel in Williamson County: acreage, parcel id, address and who publishes the map. No sign-in.

Updated

FranklinJohnsonSaline Marion Census outline · seat marked

Where it is

Williamson 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 CountyMercer 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 CountyWinnebago CountyWoodford CountyWilliamson County Williamson
Counties of Illinois · Williamson highlighted

History

Formed in 1839 from Franklin County

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

Williamson County was created from Franklin County on August 5, 1839.

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

  1. 1839
    Williamson County created Williamson created from Franklin.
    from Franklin
    · Ill. Laws 1838-1839, pp. 110-113

Land and parcels

What the maps cover in Williamson

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

Williamson by the Census

Population
67,153 2020 Census count
Housing units
31,818 2020 Census count
Land area
420 square miles
Water area
24 square miles
People per sq mi
160 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.

GIS and maps

County government

See your own lot on these maps

Open Williamson 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 Williamson County

When was Williamson County founded?
Williamson County was founded in 1839, formed from Franklin County, and named after Hugh Williamson.
What is the county seat of Williamson County?
The county seat of Williamson County is Marion.
Is there an online parcel map for Williamson County?
Not that we have found yet. Lot lines and owners of record are kept by the county assessor's office. When a public parcel map appears, it shows here.
Has Williamson County's boundary or name changed?
Williamson County was created in 1839 from Franklin County. The Atlas of Historical County Boundaries records no boundary change since.
Is my lot in a flood zone?
We do not have flood maps for Williamson 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