County Maps US

Warren County, Illinois

A county in Illinois, ranked 65 of 102 by population.

CountyFIPS 17187

Within Illinois
Warren County, highlighted, with the areas around it. Boundaries generalised from the 2025 Census cartographic boundary file; neighbour fills carry no data. The dot marks Monmouth, the county seat, located via the 2025 Census Gazetteer.

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Population
16,222
-615 (-3.7%) since 2020
Land area
542.4
square miles. A further 0.1% of the total area is water
Population density
29.9
people per square mile, computed rather than published
County seat
Monmouth
Seat of county government.
Established
1825
200 years ago
Largest city
Monmouth city
Population 8,599.

Sources: US Census Bureau, Population Estimates Program, Vintage 2025 Population Estimates, released 26 March 2026; US Census Bureau, Gazetteer Files, 2025 Gazetteer Files, released 10 September 2025; Wikidata (CC0), Wikidata. Retrieved 30 August 2026.

Satellite view

Aerial imagery of Warren County with its boundary drawn on top. A map shows you the shape; this shows you what is inside it, and which side of the line a place falls on.

Aerial view of Warren County, Illinois from the USGS National Map, with its boundary outlined
Imagery: USGS National Map, USGS Imagery Only, public domain, requested from USGS by your browser when you reach this point on the page, at 515 by 800 pixels. The boundary is the 2025 Census cartographic boundary file, reprojected to Web Mercator to match the imagery. You can also open the USGS National Map viewer.

Where Warren County ranks

Position among the 102 counties of Illinois, and among all 3,144 US counties and county-equivalents. Computed from the figures on this page.

MeasureValueIn IllinoisNationally
Population16,22265th of 1022,023rd of 3,144
Land area542.4 sq mi47th of 1021,922nd of 3,144
Population density29.9 per sq mi72nd of 1021,928th of 3,144
Median household income$67,38549th of 1021,398th of 3,143

Full data for Warren County

Every figure below is attributed to the Census Bureau product it came from, with the vintage of that product.
MeasureValueSource and vintage
Population16,222Vintage 2025 Population Estimates
Population in 202016,837Vintage 2025 Population Estimates
2020 estimates base?16,844Vintage 2025 Population Estimates
Land area542.4 sq mi2025 Gazetteer Files
Water area0.67 sq mi2025 Gazetteer Files
Total area543 sq mi2025 Gazetteer Files
Population density?29.9 per sq miComputed: Vintage 2025 estimates divided by 2025 Gazetteer land area
Median household income$67,385American Community Survey 2020-2024 5-Year Estimates
Median home value$105,000American Community Survey 2020-2024 5-Year Estimates
Median age38.5American Community Survey 2020-2024 5-Year Estimates
Households6,787American Community Survey 2020-2024 5-Year Estimates
Housing units7,587American Community Survey 2020-2024 5-Year Estimates

Sources: US Census Bureau, Population Estimates Program, Vintage 2025 Population Estimates, released 26 March 2026; US Census Bureau, Gazetteer Files, 2025 Gazetteer Files, released 10 September 2025; US Census Bureau, American Community Survey, American Community Survey 2020-2024 5-Year Estimates, released 29 January 2026. Population density is computed here, not published: it divides the Vintage 2025 population estimate by the 2025 Gazetteer land area, which are two different Census products. Retrieved 30 August 2026.

How the population changed

The Census Bureau publishes the components behind an estimate, not just the total. For the year to July 2025:

Births minus deaths
-12
178 births, 190 deaths
Net migration
+120
More people moved in than moved out.

Source: US Census Bureau, Population Estimates Program, Vintage 2025 Population Estimates, released 26 March 2026. Retrieved 30 August 2026.

The 5 areas bordering Warren County

Ordered by the length of the border actually shared, which the Census Bureau publishes in its county adjacency file and which no other county reference presents.

Bordering areaStateShared borderPopulation
Henderson CountyIllinois30 mi5,984
Knox CountyIllinois24 mi47,767
McDonough CountyIllinois18 mi25,916
Mercer CountyIllinois18 mi15,229
Fulton CountyIllinois6.0 mi32,901

Source: US Census Bureau, County Adjacency File, 2025 County Adjacency File, released 15 September 2025. Retrieved 30 August 2026.

All 102 counties of Illinois

Open the interactive Illinois county map, or jump straight to any of them.

Data on this page last rebuilt 30 August 2026. How these figures are produced.