Carroll County is a county in Illinois, United States, about 30 miles southeast of Dubuque, Iowa. It had a population of 15,702 at the 2020 census, an estimated 15,576 by 2024, and covers 466 square miles (1,210 km2) centred near 42°04′N 89°55′W.
Mount Carroll is the county seat; its postal code is 61053. The county holds 3 cities, 4 incorporated villages and 8 smaller communities across 12 townships, and it runs on Central Time. Its FIPS code is 17015.
Towns & Villages in Carroll County
Carroll County has 15 named places on record: 3 cities, 4 incorporated villages, 1 census-designated place and 7 unincorporated communities with no local government of their own. CDP stands for census designated place; Unincorp. covers crossroads and rural settlements with no formal boundary.
| Place | Type | Township | Pop. | ZIP |
|---|---|---|---|---|
| Savanna | City | Savanna | 2,783 | 61074 |
| Thomson | Village | York | 1,610 | 61285 |
| Lanark | City | — | 1,504 | 61046 |
| Mount Carroll | City · seat | Mount Carroll | 1,479 | 61053 |
| Milledgeville | Village | Wysox | 1,026 | 61051 |
| Lake Carroll | CDP | Freedom & Cherry Grove-Shannon | 852 | 61046 |
| Shannon | Village | Cherry Grove-Shannon | 801 | 61078 |
| Chadwick | Village | Fairhaven | 481 | 61014 |
| Argo Fay | Unincorp. | — | — | — |
| Arnold | Unincorp. | — | — | — |
| Ashdale Junction | Unincorp. | — | — | — |
| Blackhawk | Unincorp. | — | — | — |
| Hazelhurst | Unincorp. | — | — | — |
| Marcus | Unincorp. | — | — | — |
| Wacker | Unincorp. | — | — | — |
Thomson, a village, outranks the city of Lanark in population — its count is inflated by the more than 100 inmates counted as residents of the United States Penitentiary just north of town. Lake Carroll straddles two townships, Freedom and Cherry Grove-Shannon, because its lake and housing stock were platted across the township line rather than inside one. The county’s older place lists sometimes duplicated the Lake Carroll entry and named a dozen more hamlets with no population, ZIP, or independent record behind them; this list keeps only the seven unincorporated communities the U.S. Census Bureau’s own county profile names.
Townships
Carroll County has 12 civil townships. Only two publish an individual 2020 township population; the rest are counted in the county total but not broken out separately in public township-level tables.
| Township | Pop. (2020) | Main settlements |
|---|---|---|
| Salem | 350 | rural, no incorporated town |
| Woodland | 292 | rural, no incorporated town |
| Cherry Grove-Shannon | — | Shannon, part of Lake Carroll |
| Elkhorn Grove | — | rural, no incorporated town |
| Fairhaven | — | Chadwick |
| Freedom | — | part of Lake Carroll |
| Mount Carroll | — | Mount Carroll |
| Rock Creek-Lima | — | rural, no incorporated town |
| Savanna | — | Savanna |
| Washington | — | rural, no incorporated town |
| Wysox | — | Milledgeville |
| York | — | Thomson |
ZIP Codes
- 61053 — Mount Carroll
- 61074 — Savanna
- 61046 — Lanark (also serves Lake Carroll)
- 61285 — Thomson
- 61051 — Milledgeville
- 61078 — Shannon
- 61014 — Chadwick
Argo Fay, Arnold, Ashdale Junction, Blackhawk, Hazelhurst, Marcus and Wacker have no ZIP code of their own; mail addressed to them is sorted under the nearest incorporated town’s code.
Geography
Carroll County sits in Illinois’s northwestern corner, a stretch of bluff-and-ridge country carved by the Mississippi River, which forms the county’s entire western border. Of its 466 square miles, 445 are land and 22 are water — about 4.6 percent, most of it the river itself plus the man-made Lake Carroll. The Plum River, a Mississippi tributary, meets the big river at Savanna, and the bluffs above that confluence rise well above the surrounding farmland, unusual for this part of Illinois.
Stephenson County lies to the northeast, Ogle County to the east, Whiteside County to the south, and Jo Daviess County to the northwest; across the Mississippi, Clinton County and Jackson County in Iowa border the county to the southwest and west. Stephenson County, Ogle County, Whiteside County, Jo Daviess County, Clinton County, Iowa and Jackson County, Iowa all border Carroll directly.
Demographics
Carroll County had a population of 15,702 at the 2020 census, and the latest estimate puts the county near 15,576 residents as of 2024, a slight decline typical of rural northwestern Illinois. Savanna and Thomson are the two largest communities, though Thomson’s count includes the federal prison population housed there. Median household income runs $60,758 (2024 estimate), below Illinois’s statewide figure of $80,306 (2023 ACS). Population density works out to roughly 35 people per square mile of land, low even by rural-Illinois standards.
Economy & Local Life
Manufacturing employs the most workers in the county, about 1,456 people, followed by health care and social assistance at 834 and retail trade at 605. The former Savanna Army Depot, a 13,000-acre munitions storage and maintenance installation that operated from World War II through the Vietnam era before closing in 2000, is now Savanna Depot Park, home to logistics, recycling and railcar businesses. In Mount Carroll, the Campbell Center for Historic Preservation Studies occupies the former Shimer College campus, running preservation-trades training programs on the site. Thomson’s biggest single employer is the United States Penitentiary, a federal facility the Bureau of Prisons bought from the state in 2012 and has been converting to maximum-security standards.
History
The Illinois General Assembly created Carroll County from Jo Daviess County in 1839. It was named for Charles Carroll of Carrollton, the last surviving signer of the Declaration of Independence, who died in 1832. Mount Carroll became the county seat, and its courthouse still anchors the Mount Carroll Historic District, a 118-acre area added to the National Register of Historic Places in 1980.
Savanna grew around river trade on the Mississippi after its founding in 1828, then took on a military role in 1917 when the U.S. Army established what became Savanna Army Depot. On 31 January 2019, Mount Carroll recorded a low of −38°F, the coldest temperature ever measured in Illinois.
Getting There
No interstate highway crosses Carroll County. US Route 52 runs along the Mississippi through Savanna, and Illinois Routes 40, 64, 72, 73, 78 and 84 cross the county’s farmland and river bluffs.
- Freeport — about 25 miles east
- Sterling-Rock Falls — about 25 miles southeast
- Dubuque, Iowa — about 30 miles northwest
- Clinton, Iowa — about 20 miles west
Quad Cities International Airport, in Moline, is the nearest major commercial airport, roughly 50 miles south. BNSF and Canadian Pacific freight lines run through Savanna, but no Amtrak passenger service reaches the county.
Frequently Asked Questions
Where is Carroll County located?
It sits in Illinois’s northwestern corner along the Mississippi River, about 30 miles southeast of Dubuque, Iowa, bordered by Stephenson, Ogle, Whiteside and Jo Daviess counties and, across the river, Clinton and Jackson counties in Iowa.
What is the population of Carroll County?
15,702 at the 2020 census, with a 2024 estimate near 15,576.
How big is Carroll County?
466 square miles (1,210 km2), of which 445 square miles are land.
What is the county seat of Carroll County?
Mount Carroll, home to the Mount Carroll Historic District and the county courthouse.
How many towns and villages are in Carroll County?
Three cities — Savanna, Lanark and Mount Carroll — four villages — Thomson, Milledgeville, Shannon and Chadwick — the Lake Carroll census-designated place, and seven unincorporated communities.
What ZIP codes does Carroll County use?
61053 for Mount Carroll, 61074 for Savanna, 61046 for Lanark and Lake Carroll, 61285 for Thomson, 61051 for Milledgeville, 61078 for Shannon and 61014 for Chadwick.
What is Carroll County known for?
Its namesake Charles Carroll of Carrollton, the Mississippi River bluffs at Savanna, the former Savanna Army Depot, and the federal penitentiary at Thomson.
More counties in this state: Towns and villages in Illinois.