Williamson County area

Population and density of Williamson County

The Census Bureau estimates that 67,225 people lived in Williamson County on July 1, 2025. That ranks #825 of 3,222 county equivalents nationally and #22 of 102 in Illinois. Since the April 1, 2020 estimates base of 67,163, the population has grown by 62 (+0.1%), compared with −0.8% for Illinois as a whole.

With 420.2 square miles of land, that is 160 people per square mile (62 per km²), placing Williamson County #746 nationally for density and #19 in Illinois. The median county equivalent has 47 people per square mile; the Illinois median is 44. Put another way, there are about 4 acres of land per resident.

U.S. Census Bureau, Vintage 2025 population estimates. Estimates are revised each year and are not decennial census counts. Density divides the whole population by the whole county’s land area, so it describes an average, not how settlement is distributed within the county. Population sources and methods.

Cities and towns in Williamson County

16 incorporated places lie at least partly in Williamson County. Together they are home to 45,504 people (67.7%); the other 21,721 (32.3%) live outside any incorporated place, in unincorporated communities and rural areas.

The largest is Marion city, with 17,148 residents in the county. By land, Carbondale city is the largest at 17.5 mi² across all its counties. Among places with at least 1,000 residents in 2020, Cambria village grew fastest since then (+20.2%).

Population: U.S. Census Bureau Vintage 2025 subcounty estimates, counting only the part of each place inside Williamson County. Land: 2026 Census Gazetteer, for the whole place even where it extends into other counties. How communities are listed.

Metropolitan area

Williamson County is a central county of the Marion-Herrin, IL Micropolitan Statistical Area. Williamson County is the area’s only county equivalent, so the micro area has the same 420.2 square miles of land. By combined land it ranks #470 of 542 micropolitan areas, and #123 by population. It belongs to the Carbondale-Marion-Herrin, IL Combined Statistical Area.

ZIP codes

22 ZIP Code Tabulation Areas have land in Williamson County; 12 of them have most of their land inside the county, and the others mainly cover neighboring areas.

62841628906289662901629026291562917629186292162922629246293362935629396294862949629516295862959629676297462987

ZIP Code Tabulation Areas are the Census Bureau’s approximations of USPS ZIP Code service areas, from the 2020 Census relationship files. Lighter codes have less than half of their land in this county. Post-office-box and single-address ZIP Codes have no tabulation area.

Counties bordering Williamson County

Williamson County shares a land border with 5 county equivalents. It has more land than 4 of them. The largest neighbor is Jackson County (583.6 mi²) and the smallest is Johnson County (343.7 mi²). Together with its neighbors, Williamson County forms a block of 2,549.9 mi² of land, of which it holds 16.5%.

How big is Williamson County compared with U.S. states?

Williamson County has less land than any of the 50 states. Rhode Island, the smallest state by land area, has 1,033.9 mi², about 2.5 times as much. Illinois’s county land could hold about 132 areas the size of Williamson County.

Another way to picture it: the same land area would fill a square about 20.5 miles (33 km) on each side. Real county shapes are rarely square, so this is a scale reference rather than a width or driving distance.

State figures are the sums of each state’s county-equivalent land measurements in the same Census release, so states and counties are compared on the same basis. Published state totals can differ slightly because of rounding. Compare all states · Counties bigger than entire states.

What stands out about Williamson County?

Williamson County has more land than 810 of the other 3,221 county equivalents in the dataset.

A median is the middle county-equivalent measurement, not the average of the largest and smallest. All three bars use the same linear scale. Very small areas may appear as a thin sliver; the labels retain their values.

Does including water change its rank?

Using land plus water places Williamson County at #2,367 nationally by total area, 45 places closer to the top than its land rank. The water assigned to this geography moves it ahead of some county equivalents with more land.

Its water share is 5.4%, compared with a national county-equivalent median of 1.2% — 4.3 percentage points higher. This describes the statistical geography’s land/water mix, not flood exposure or developable land.

Closest positions in the national land ranking

These entries sit immediately beside Williamson County in the national ranking. They can be in different states and are not necessarily geographic neighbors.

Calculated from the same Census release as the area table. Total-area order uses original ALAND + AWATER; water shares and chart labels use published square miles. Calculation details.