Adams County	001	Quincy	1825	Pike County	John Quincy Adams (1767.1848), sixth President of the United States	67,103	857 sq mi
(2,220 km2)	State map highlighting Adams County
Alexander County	003	Cairo	1819	Union County	William M. Alexander, settler and state representative in the Illinois General Assembly	8,238	236 sq mi
(611 km2)	State map highlighting Alexander County
Bond County	005	Greenville	1817	Crawford County, Edwards County, and Madison County	Shadrach Bond (1773.1832), first Governor of Illinois	17,768	380 sq mi
(984 km2)	State map highlighting Bond County
Boone County	007	Belvidere	1837	Winnebago County	Daniel Boone (1734.1820), trailblazer of the Wilderness Road in Kentucky	54,165	281 sq mi
(728 km2)	State map highlighting Boone County
Brown County	009	Mount Sterling	1839	Schuyler County	Jacob Brown (1775.1828), successful War of 1812 army officer responsible for Great Lakes defenses	6,937	306 sq mi
(793 km2)	State map highlighting Brown County
Bureau County	011	Princeton	1837	Putnam County	Pierre de Bureo, Frenchman, North American fur trader	34,978	869 sq mi
(2,251 km2)	State map highlighting Bureau County
Calhoun County	013	Hardin	1825	Pike County	John C. Calhoun (1782.1850), South Carolina senator and seventh Vice President of the United States	5,089	254 sq mi
(658 km2)	State map highlighting Calhoun County
Carroll County	015	Mount Carroll	1839	Jo Daviess	Charles Carroll of Carrollton (1737.1832), signed the Declaration of Independence on behalf of Maryland	15,387	444 sq mi
(1,150 km2)	State map highlighting Carroll County
Cass County	017	Virginia	1837	Morgan County	Lewis Cass (1782.1866), second governor of Michigan Territory, fourteenth United States Secretary of War	13,642	376 sq mi
(974 km2)	State map highlighting Cass County
Champaign County	019	Urbana	1833	Vermilion County	Champaign County, Ohio, which took its name from the French for "open level country"	201,081	997 sq mi
(2,582 km2)	State map highlighting Champaign County
Christian County	021	Taylorville	1839	Sangamon County	Christian County, Kentucky, which was itself named after Colonel William Christian	34,800	709 sq mi
(1,836 km2)	State map highlighting Christian County
Clark County	023	Marshall	1819	Crawford County	George Rogers Clark (1752.1818), highest-ranking officer in the Northwest Territory during the American Revolution	16,335	502 sq mi
(1,300 km2)	State map highlighting Clark County
Clay County	025	Louisville	1824	Wayne, Lawrence, Fayette, and Crawford County	Henry Clay (1777.1852), Kentucky legislator who negotiated the Missouri Compromise	13,815	469 sq mi
(1,215 km2)	State map highlighting Clay County
Clinton County	027	Carlyle	1824	Washington, Bond, and Fayette County	DeWitt Clinton (1769.1828), Governor of New York, responsible for the construction of the Erie Canal	37,762	474 sq mi
(1,228 km2)	State map highlighting Clinton County
Coles County	029	Charleston	1830	Clark and Edgar County	Edward Coles (1786.1868), second Governor of Illinois, responsible for the abolition of slavery in Illinois	53,873	508 sq mi
(1,316 km2)	State map highlighting Coles County
Cook County	031	Chicago	1831	Putnam County	Daniel Pope Cook (1794.1827), politician and first Attorney General of Illinois	5,194,675	946 sq mi
(2,450 km2)	State map highlighting Cook County
Crawford County	033	Robinson	1816	Edwards County	William H. Crawford (1772.1834), ninth United States Secretary of War, seventh Secretary of the Treasury	19,817	444 sq mi
(1,150 km2)	State map highlighting Crawford County
Cumberland County	035	Toledo	1843	Coles County	Disputed: Cumberland Road, which entered the county; Cumberland, Maryland; or Cumberland River in Kentucky	11,048	346 sq mi
(896 km2)	State map highlighting Cumberland County
DeKalb County	037	Sycamore	1837	Kane County	Johann de Kalb (1721.1780), German soldier in the Continental Army who fought alongside Gilbert du Motier, marquis de Lafayette	105,160	634 sq mi
(1,642 km2)	State map highlighting DeKalb County
DeWitt County	039	Clinton	1839	Macon and McLean County	DeWitt Clinton (1769.1828), Governor of New York, responsible for the construction of the Erie Canal	16,561	398 sq mi
(1,031 km2)	State map highlighting DeWitt County
Douglas County	041	Tuscola	1859	Coles County	Stephen A. Douglas (1813.61), prominent Illinois Democrat who engaged in debates with Abraham Lincoln	19,980	417 sq mi
(1,080 km2)	State map highlighting Douglas County
DuPage County	043	Wheaton	1839	Cook County	DuPage River	916,924	334 sq mi
(865 km2)	State map highlighting DuPage County
Edgar County	045	Paris	1823	Clark County	John Edgar (c. 1750.1832), Illinois delegate to the Northwest Territory legislature; at the time, wealthiest man in Illinois	18,576	624 sq mi
(1,616 km2)	State map highlighting Edgar County
Edwards County	047	Albion	1814	Gallatin County and Madison County	Ninian Edwards (1775.1833), third Governor of the State of Illinois and only governor of the Illinois Territory	6,721	222 sq mi
(575 km2)	State map highlighting Edwards County
Effingham County	049	Effingham	1831	Fayette and Crawford County	Thomas Howard, 3rd Earl of Effingham, military officer who resigned from the British Army to avoid fighting the American colonies	34,242	479 sq mi
(1,241 km2)	State map highlighting Effingham County
Fayette County	051	Vandalia	1821	Bond, Wayne, Clark, Jefferson, and Crawford County	Gilbert du Motier, marquis de Lafayette (1757.1834), French military officer who was a key factor in the American and French Revolutions.	22,140	716 sq mi
(1,854 km2)	State map highlighting Fayette County
Ford County	053	Paxton	1859	Vermilion County	Thomas Ford (1800.50), eighth Governor of Illinois; served during the Illinois Mormon War	14,081	486 sq mi
(1,259 km2)	State map highlighting Ford County
Franklin County	055	Benton	1818	White County and Gallatin County	Benjamin Franklin (1706.90), prolific writer, inventor, and politician; key factor in the American Revolution	39,561	412 sq mi
(1,067 km2)	State map highlighting Franklin County
Fulton County	057	Lewistown	1823	Pike County	Robert Fulton (1765.1815), inventor of the steamboat	37,069	866 sq mi
(2,243 km2)	State map highlighting Fulton County
Gallatin County	059	Shawneetown	1812	Randolph County	Albert Gallatin (1761.1849), fourth and longest-serving United States Secretary of the Treasury	5,589	324 sq mi
(839 km2)	State map highlighting Gallatin County
Greene County	061	Carrollton	1821	Madison County	Nathanael Greene (1742.86), major general in the Continental Army	13,886	543 sq mi
(1,406 km2)	State map highlighting Greene County
Grundy County	063	Morris	1841	LaSalle County	Felix Grundy (1777.1840), Tennessean senator who served as the thirteenth United States Attorney General	50,063	420 sq mi
(1,088 km2)	State map highlighting Grundy County
Hamilton County	065	McLeansboro	1821	White County	Alexander Hamilton (1755.1804), first United States Secretary of the Treasury	8,457	435 sq mi
(1,127 km2)	State map highlighting Hamilton County
Hancock County	067	Carthage	1825	Adams County	John Hancock (1737.93), first governor of the Massachusetts colony and president of the Second Continental Congress	19,104	795 sq mi
(2,059 km2)	State map highlighting Hancock County
Hardin County	069	Elizabethtown	1839	Pope County	Hardin County, Kentucky, which was itself named after John Hardin	4,320	178 sq mi
(461 km2)	State map highlighting Hardin County
Henderson County	071	Oquawka	1841	Warren County	Henderson County, Kentucky, which was itself named after Richard Henderson	7,331	379 sq mi
(982 km2)	State map highlighting Henderson County
Henry County	073	Cambridge	1825	Fulton County	Patrick Henry (1736-99), American Revolutionary War figure; first and sixth Governor of Virginia	50,486	823 sq mi
(2,132 km2)	State map highlighting Henry County
Iroquois County	075	Watseka	1833	Vermilion County	Iroquois Native Americans	29,718	1,116 sq mi
(2,890 km2)	State map highlighting Iroquois County
Jackson County	077	Murphysboro	1816	Randolph County and Johnson County	Andrew Jackson (1767.1845), seventh President of the United States, United States Senator from Tennessee, and general in the War of 1812	60,218	588 sq mi
(1,523 km2)	State map highlighting Jackson County
Jasper County	079	Newton	1831	Clay and Crawford County	Sgt. William Jasper (c. 1750-79), American Revolutionary War soldier popularized by Parson Weems	9,698	494 sq mi
(1,279 km2)	State map highlighting Jasper County
Jefferson County	081	Mount Vernon	1819	Edwards and White County	Thomas Jefferson (1743.1826), third President of the United States, second Vice President of the United States, Governor of Virginia, and one of the foremost Founding Fathers of the United States	38,827	571 sq mi
(1,479 km2)	State map highlighting Jefferson County
Jersey County	083	Jerseyville	1839	Greene County	State of New Jersey, from which many early settlers hailed	22,985	369 sq mi
(956 km2)	State map highlighting Jersey County
Jo Daviess County	085	Galena	1827	Henry, Mercer, and Putnam County	Joseph Hamilton Daveiss (1774.1811), commander of the Indiana Dragoons at the Battle of Tippecanoe	22,678	601 sq mi
(1,557 km2)	State map highlighting Jo Daviess County
Johnson County	087	Vienna	1812	Randolph County	Richard Mentor Johnson (abt. 1780-1850), ninth Vice President of the United States and United States Senator from Kentucky	12,582	346 sq mi
(896 km2)	State map highlighting Johnson County
Kane County	089	Geneva	1836	LaSalle County	Elias Kane (1794.1835), United States Senator from Illinois	515,269	521 sq mi
(1,349 km2)	State map highlighting Kane County
Kankakee County	091	Kankakee	1853	Iroquois and Will County	Kankakee River	113,449	678 sq mi
(1,756 km2)	State map highlighting Kankakee County
Kendall County	093	Yorkville	1841	LaSalle and Kane County	Amos Kendall (1789.1869), United States Postmaster General under Presidents Andrew Jackson and Martin Van Buren	114,736	321 sq mi
(831 km2)	State map highlighting Kendall County
Knox County	095	Galesburg	1825	Fulton County	Gen. Henry Knox (1750.1806), American Revolutionary War general and first United States Secretary of War	52,919	716 sq mi
(1,854 km2)	State map highlighting Knox County
Lake County	097	Waukegan	1839	McHenry County	Lake Michigan	703,462	448 sq mi
(1,160 km2)	State map highlighting Lake County
LaSalle County	099	Ottawa	1831	Putnam and Tazewell County	Sieur de la Salle (1643-87), French explorer of the Great Lakes	113,924	1,135 sq mi
(2,940 km2)	State map highlighting LaSalle County
Lawrence County	101	Lawrenceville	1821	Crawford and Edwards County	Capt. James Lawrence (1781.1813), commander of the USS Chesapeake in the War of 1812. Famous for his command "Don't give up the ship!"	16,833	372 sq mi
(963 km2)	State map highlighting Lawrence County
Lee County	103	Dixon	1839	Ogle County	"Light Horse" Henry Lee III (1756.1818), American Revolutionary War officer and ninth Governor of Virginia	36,031	725 sq mi
(1,878 km2)	State map highlighting Lee County
Livingston County	105	Pontiac	1837	LaSalle and McLean County	Edward Livingston (1764.1836), prominent jurist, Congressman from New York and Louisiana, and U.S. Secretary of State from 1831-33	38,950	1,044 sq mi
(2,704 km2)	State map highlighting Livingston County
Logan County	107	Lincoln	1839	Sangamon County	John Logan, country doctor and early settler, and the father of John A. Logan	30,305	618 sq mi
(1,601 km2)	State map highlighting Logan County
Macon County	115	Decatur	1829	Shelby County	Nathaniel Macon (1758.1837), sixth Speaker of the United States House of Representatives and United States Senator from North Carolina	110,768	581 sq mi
(1,505 km2)	State map highlighting Macon County
Macoupin County	117	Carlinville	1829	Greene County	Native American word macoupin, meaning "American lotus"	47,765	864 sq mi
(2,238 km2)	State map highlighting Macoupin County
Madison County	119	Edwardsville	1812	St. Clair County and Randolph County	James Madison (1751.1836), fourth President of the United States and principal author of the Constitution of the United States	269,282	725 sq mi
(1,878 km2)	State map highlighting Madison County
Marion County	121	Salem	1823	Fayette and Jefferson County	Francis Marion (c. 1732-95), general in the American Revolutionary War known as "The Swamp Fox"	39,437	572 sq mi
(1,481 km2)	State map highlighting Marion County
Marshall County	123	Lacon	1839	Putnam County	John Marshall (1755.1835), fourth and longest-serving Chief Justice of the United States, wrote opinion in Marbury v. Madison establishing the principle of judicial review	12,640	386 sq mi
(1,000 km2)	State map highlighting Marshall County
Mason County	125	Havana	1841	Tazewell and Menard County	Named after Mason County, Kentucky, itself named after George Mason	14,666	539 sq mi
(1,396 km2)	State map highlighting Mason County
Massac County	127	Metropolis	1843	Pope and Johnson County	Fort Massac, a colonial-era fort on the Ohio River	15,429	239 sq mi
(619 km2)	State map highlighting Massac County
McDonough County	109	Macomb	1826	Schuyler County	Commodore Thomas Macdonough (1783.1825), commander of American naval forces at the Battle of Plattsburgh	32,612	589 sq mi
(1,526 km2)	State map highlighting McDonough County
McHenry County	111	Woodstock	1836	Cook and LaSalle County	Major William McHenry (c. 1771 . 1835), officer in during several campaigns against Native Americans and member of the Illinois legislature	308,760	604 sq mi
(1,564 km2)	State map highlighting McHenry County
McLean County	113	Bloomington	1830	Tazewell County	John McLean (1791.1830), United States Representative and United States Senator from Illinois (the latter from 1824-25 and 1829-30)	169,572	1,184 sq mi
(3,067 km2)	State map highlighting McLean County
Menard County	129	Petersburg	1839	Sangamon County	Pierre Menard (1766.1844), prominent early settler and first Lieutenant Governor of Illinois	12,705	314 sq mi
(813 km2)	State map highlighting Menard County
Mercer County	131	Aledo	1825	Schuyler County	Hugh Mercer (1726-77), British officer in the Seven Years' War and general in the Continental Army during the American Revolutionary War	16,434	561 sq mi
(1,453 km2)	State map highlighting Mercer County
Monroe County	133	Waterloo	1816	Randolph County and St. Clair County	James Monroe (1758.1831), seventh United States Secretary of State, eighth United States Secretary of War, Governor of Virginia, and fifth President of the United States	32,957	388 sq mi
(1,005 km2)	State map highlighting Monroe County
Montgomery County	135	Hillsboro	1821	Bond and Madison County	Gen. Richard Montgomery (1738-75), brigadier-general in the Continental Army who led the unsuccessful invasion of Canada	30,104	704 sq mi
(1,823 km2)	State map highlighting Montgomery County
Morgan County	137	Jacksonville	1823	Sangamon County	Gen. Daniel Morgan (1736.1802), successful tactician in the American Revolutionary War and later United States Representative from Virginia	35,547	569 sq mi
(1,474 km2)	State map highlighting Morgan County
Moultrie County	139	Sullivan	1843	Shelby and Macon County	Gen. William Moultrie (1730.1805), American Revolutionary War general and Governor of South Carolina	14,846	336 sq mi
(870 km2)	State map highlighting Moultrie County
Ogle County	141	Oregon	1836	Jo Daviess	Joseph Ogle (1737.1821), early settler in southwest Illinois, who helped found the first Methodist church in Illinois	53,497	759 sq mi
(1,966 km2)	State map highlighting Ogle County
Peoria County	143	Peoria	1825	Fulton County	The Peoria Native American tribe	186,494	620 sq mi
(1,606 km2)	State map highlighting Peoria County
Perry County	145	Pinckneyville	1827	Randolph and Jackson County	Commodore Oliver Hazard Perry (1785.1819), American naval officer who led the victorious American forces at the Battle of Lake Erie	22,350	441 sq mi
(1,142 km2)	State map highlighting Perry County
Piatt County	147	Monticello	1841	DeWitt and Macon County	John Piatt, the patriarch of a prominent settler family in the early history of the county	16,729	440 sq mi
(1,140 km2)	State map highlighting Piatt County
Pike County	149	Pittsfield	1821	Madison, Bond, and Clark County	Zebulon Pike (1779.1813), early explorer of the American Southwest, namesake of Pikes Peak	16,430	830 sq mi
(2,150 km2)	State map highlighting Pike County
Pope County	151	Golconda	1816	Gallatin and Johnson County	Nathaniel Pope (1784.1850), early Delegate from Illinois Territory to Congress and judge on the United States District Court for the District of Illinois	4,470	371 sq mi
(961 km2)	State map highlighting Pope County
Pulaski County	153	Mound City	1843	Alexander and Johnson County	Gen. Casimir Pulaski (1745-79), Polish American general of cavalry in the American Revolutionary War	6,161	201 sq mi
(521 km2)	State map highlighting Pulaski County
Putnam County	155	Hennepin	1825	Fulton County	Gen. Israel Putnam (1718-90), commander of American forces at the Battle of Bunker Hill	6,006	160 sq mi
(414 km2)	State map highlighting Putnam County
Randolph County	157	Chester	1795	St. Clair County	Edmund Randolph (1753.1813), first Attorney General of the United States, and briefly United States Secretary of State	33,476	578 sq mi
(1,497 km2)	State map highlighting Randolph County
Richland County	159	Olney	1841	Clay and Lawrence County	Richland County, Ohio, itself named for its rich soil	16,233	360 sq mi
(932 km2)	State map highlighting Richland County
Rock Island County	161	Rock Island	1831	Jo Daviess County	Rock Island	147,546	427 sq mi
(1,106 km2)	State map highlighting Rock Island County
Saline County	165	Harrisburg	1847	Gallatin County	Salt springs within the county	24,913	383 sq mi
(992 km2)	State map highlighting Saline County
Sangamon County	167	Springfield	1821	Madison and Bond County	Sangamon River	197,465	868 sq mi
(2,248 km2)	State map highlighting Sangamon County
Schuyler County	169	Rushville	1825	Pike and Fulton County	Gen. Philip Schuyler (1733.1804), American Revolutionary War general and United States Senator from New York	7,544	437 sq mi
(1,132 km2)	State map highlighting Schuyler County
Scott County	171	Winchester	1839	Morgan County	Scott County, Kentucky, itself named after Charles Scott	5,355	251 sq mi
(650 km2)	State map highlighting Scott County
Shelby County	173	Shelbyville	1827	Fayette County	Isaac Shelby (1750.1826), soldier in the American Revolutionary War and War of 1812; first and fifth Governor of Kentucky	22,363	759 sq mi
(1,966 km2)	State map highlighting Shelby County
St. Clair County	163	Belleville	1790	original two counties	Arthur St. Clair (1737.1818), major general in the American Revolutionary War and first Governor of the Northwest Territory	270,056	664 sq mi
(1,720 km2)	State map highlighting St. Clair County
Stark County	175	Toulon	1839	Knox and Putnam County	Gen. John Stark (1728.1822), general in the American Revolutionary War, called the "Hero of Bennington"	5,994	288 sq mi
(746 km2)	State map highlighting Stark County
Stephenson County	177	Freeport	1837	Jo Daviess and Winnebago County	Benjamin Stephenson (1769.1822), representative of Illinois Territory in the United States Congress from 1814 to 1816	47,711	564 sq mi
(1,461 km2)	State map highlighting Stephenson County
Tazewell County	179	Pekin	1827	Sangamon County	Littleton Waller Tazewell (1774.1860), United States Senator from (and later governor of) Virginia	135,394	649 sq mi
(1,681 km2)	State map highlighting Tazewell County
Union County	181	Jonesboro	1818	Johnson County	The federal union of the states	17,808	416 sq mi
(1,077 km2)	State map highlighting Union County
Vermilion County	183	Danville	1826	Edgar County	The Vermilion River	81,625	899 sq mi
(2,328 km2)	State map highlighting Vermilion County
Wabash County	185	Mount Carmel	1824	Edwards County	The Wabash River	11,947	224 sq mi
(580 km2)	State map highlighting Wabash County
Warren County	187	Monmouth	1825	Schuyler County	Joseph Warren (1741-75), played a role in American Patriot movements, a prominent early fatality in the American Revolutionary War	17,707	543 sq mi
(1,406 km2)	State map highlighting Warren County
Washington County	189	Nashville	1818	St. Clair County	George Washington (1732-99), commander-in-chief of American forces in the American Revolutionary War and first President of the United States	14,716	563 sq mi
(1,458 km2)	State map highlighting Washington County
Wayne County	191	Fairfield	1819	Edwards County	Gen. "Mad" Anthony Wayne (1745-96), major general in the United States Army in the American Revolutionary War and the Northwest Indian War	16,760	714 sq mi
(1,849 km2)	State map highlighting Wayne County
White County	193	Carmi	1815	Gallatin County	Isaac White (1776.1811), resident of Illinois who enlisted in the Indiana militia and was killed at the Battle of Tippecanoe	14,665	495 sq mi
(1,282 km2)	State map highlighting White County
Whiteside County	195	Morrison	1836	Jo Daviess and Henry County	Samuel Whiteside (1783.1868), state legislator and militia leader	58,498	685 sq mi
(1,774 km2)	State map highlighting Whiteside County
Will County	197	Joliet	1836	Cook and Iroquois County	Conrad Will (1779.1835),[5] physician, local businessman and longtime member of the state legislature	677,560	837 sq mi
(2,168 km2)	State map highlighting Will County
Williamson County	199	Marion	1839	Franklin County	Hugh Williamson (1735.1819), delegate from North Carolina to the Philadelphia Convention	66,357	424 sq mi
(1,098 km2)	State map highlighting Williamson County
Winnebago County	201	Rockford	1836	Jo Daviess	Winnebago Native Americans	295,266	514 sq mi
(1,331 km2)	State map highlighting Winnebago County
Woodford County	203	Eureka	1841	Tazewell and McLean County	Gen. William Woodford (1734-80), brigadier general in the American Revolutionary War who died while a British prisoner	38,664	528 sq mi
(1,368 km2)	State map highlighting Woodford County