ADAMS COUNTY	Established: Jan. 13, 1825 Parent Counties: Pike County Seat: Quincy Named After: John Quincy Adams, sixth U.S. president
ALEXANDER COUNTY	Established: March 4, 1819 Parent Counties: County Seat: Cairo Named After: William M. Alexander, early settler of the district, also state representatives in second and third General Assemblies
BOND COUNTY	Established: Jan. 4, 1817 Parent Counties: Madison, Crawford, and Edwards County Seat: Greenville Named After: Gov. Shadrach Bond
BOONE COUNTY	Established: March 4, 1837 Parent Counties: Winnebago County Seat: Belvedere Named After: Daniel Boone, pioneer hunter, explorer and Indian fighter.
BROWN COUNTY	Established: Feb. 1, 1839 Parent Counties: Schuyler County Seat: Mt. Sterling Named After: Gen. Jacob Brown, soldier in War of 1812.
BUREAU COUNTY	Established: Feb. 28, 1837 Parent Counties: Putnam County Seat: Princeton Named After: Pierre de Bureo, French trader with Native Americans
CALHOUN COUNTY	Established: Jan. 10, 1825 Parent Counties: County Seat: Hardin Named After: John C. Calhoun, southern statesman and U.S. vice-president under Adams and Jackson.
CARROLL COUNTY	Established: Feb. 22, 1839 Parent Counties: Jo Daviess County Seat: Mt. Carroll Named After: Charles Carroll of Carrollton, signer of the Declaration of Independence
CASS COUNTY	Established: March 3, 1837 Parent Counties: Sangamon County Seat: Virginia Named After: Gen.Lewis Cass, secretary of war under Jackson and secretary of state under Buchanan
CHAMPAIGN COUNTY	Established: Feb. 20, 1833 Parent Counties: County Seat: Urbana Named After: a county in Ohio
CHRISTIAN (formerly DANE) COUNTY	Established: Feb. 15, 1839 Parent Counties: Sangamon County Seat: Taylorville Named After: A county in Kentucky
CLARK COUNTY	Established: March 22, 1819 Parent Counties: Crawford County Seat: Marshall Named After: George Rogers Clark, soldier of the Revolution and captor of Ft. Vincennes and Kaskaskia
CLAY COUNTY	Established: Dec. 23, 1824 Parent Counties: County Seat: Louisville Named After: Henry Clay, famed politician and author of the Missouri Compromise
CLINTON COUNTY	Established: Dec. 27, 1824 Parent Counties: Washington, Bond & Fayette County Seat: Carlyle Named After: DeWitt Clinton, mayor of the city and governor of the State of New York; promoter of the Erie Canal
COLES COUNTY	Established: Dec. 25, 1830 Parent Counties: Clark & Edgar County Seat: Charleston Named After: Gov. Edward Coles
COOK COUNTY	Established: Jan. 15, 1831 Parent Counties: Peoria County Seat: Chicago Named After: Daniel P. Cook, pioneer lawyer, first attorney general of Illinois and member of Congress from 1819 to 1827.
CRAWFORD COUNTY	Established: Dec. 31, 1816 Parent Counties: County Seat: Robinson Named After: William H. Crawford, prominent U.S. senator from Georgia, Secretary of War and candidate for president in 1824.
CUMBERLAND COUNTY	Established: March 2, 1843 Parent Counties: Coles County Seat: Toledo Named After: the Cumberland River (or from Cumberland in Maryland, source:"The Making of Illinois",1900, or from the Cumberland Road, "which as originally projected, passed through it" (source Historical Enclyopedia of IL, 1901)
DeKALB COUNTY	Established: March 4, 1837 Parent Counties: Kane County County Seat: Sycamore Named After: Joann DeKalb, German who fought in the Revolution; killed in 1870
DeWITT COUNTY	Established: March 1, 1839 Parent Counties: County Seat: Clinton Named After: Dewitt Clinton [see Clinton]
DOUGLAS COUNTY	Established: Feb. 8, 1859 Parent Counties: Coles County Seat: Tuscola Named After: Stephen A. Douglas, U.S. senator, 1847 to 1861
DuPAGE COUNTY	Established: Feb. 9, 1839 Parent Counties: County Seat: Wheaton Named After: the DuPage River
EDGAR COUNTY	Established: Jan. 3, 1823 Parent Counties: Clark County County Seat: Paris Named After: John Edgar, pioneer merchant and politician.
EDWARDS COUNTY	Established: Nov. 28, 1814 Parent Counties: Gallatin County Seat: Albion Named After: Gov. Ninian Edwards
EFFINGHAM COUNTY	Established: Feb. 15, 1831 Parent Counties: Fayette County Seat: Effingham Named After: Lord Edward Effingham, who resigned his post in the British Army rather than fight the colonies in 1775.
FAYETTE COUNTY	Established: Feb. 14, 1821 Parent Counties: Clark, Bond, Jefferson, Wayne and Crawford County Seat: Vandalia Named After: Marquis de La Fayette
FORD COUNTY	Established: Feb. 17, 1859 Parent Counties: Vermilion County Seat: Paxton Named After: Gov. Thomas Ford
FRANKLIN COUNTY	Established: Jan. 2, 1818 Parent Counties: County Seat: Benton Named After: Benjamin Franklin, famed statesman, who was U.S. ambassador to France during the Revolution.
FULTON COUNTY	Established: Jan. 28, 1823 Parent Counties: County Seat: Lewistown Named After: Robert Fulton, first successful builder of steamboats on American waters.
GALLATIN COUNTY	Established: Sept. 14, 1812 Parent Counties: St. Clair & Randolph County Seat: Shawneetown Named After: Albert Gallatin, Secretary of the Treasury, fiinancier and member of Congress from Pennsylvania.
GREENE COUNTY	Established: Jan. 20, 1821 Parent Counties: Madison County County Seat: Carrollton Named After: Gen. Nathaniel Greene, commander in southern colonies during the Revolution.
GRUNDY COUNTY	Established: Feb. 17, 1841 Parent Counties: La Salle County Seat: Morris Named After: Felix Grundy, U.S. senator from Tennessee and attorney general of the U.S.
HAMILTON COUNTY	Established: Feb. 8, 1821 Parent Counties: County Seat: McLeansboro Named After: Alexander Hamilton, revolutionary soldier and first secretary of the treasury, from 1789 to 1795
HANCOCK COUNTY	Established: Jan. 13, 1825 Parent Counties: Pike and Adams Counties County Seat: Carthage Named After: John Hancock, revolutionary soldier and first signer of Declaration of Independence.
HARDIN COUNTY	Established: Jan. 20, 1841 Parent Counties: County Seat: Elizabethtown Named After: a county in Kentucky.
HENDERSON COUNTY	Established: Jan. 20, 1841 Parent Counties: Warren County Seat: Oquawka Named After: a county in Kentucky.
HENRY COUNTY	Established: Jan. 13, 1825 Parent Counties: County Seat: Cambridge Named After: Patrick Henry, famed orator, revolutionary soldier and governor of Virginia.
IROQUOIS COUNTY	Established: Feb. 26, 1833 Parent Counties: Vermilion County Seat: Watseka Named After: A Native American Tribe
JACKSON COUNTY	Established: Jan. 10, 1816 Parent Counties: Randolph and Johnson County Seat: Murphysboro Named After: Andrew Jackson
JASPER COUNTY	Established: Feb. 15, 1831 Parent Counties: County Seat: Newton Named After: Sgt. William Jasper, revolutionary hero at Charleston and Savannah.
JEFFERSON COUNTY	Established: March 26, 1819 Parent Counties: Edwards & White County Seat: Mt. Vernon Named After: Thomas Jefferson.
JERSEY COUNTY	Established: Feb. 28, 1839 Parent Counties: Greene County Seat: Jerseyville Named After: New Jersey
JO DAVIESS	Established: Feb. 17, 1827 Parent Counties: Peoria County County Seat: Galena Named After:J oseph Hamilton Daviess, prominent Kentucky lawyer and soldier; slain at the battle of Tippecanoe in 1811
JOHNSON COUNTY	Established: Sept. 14, 1812 Parent Counties: Randolph County County Seat: Vienna Named After: Col. Richard M. Johnson, reputed to have killed the Indian Chief Tecumseh; also U.S. vice-president, 1937-41

KANE	Established: June 1836 Parent Counties: County Seat: Geneva Named After: Senator Elias Kent Kane
KANKAKEE	Established: 1851 Parent Counties: Will & Iroquois County Seat: Named After: The Kankakee River
KENDALL	Established: 1841 Parent Counties: LaSalle and Kane County Seat: Yorkville Named After: The PostMaster General under President Jackson
KNOX	Established: 1830 Parent Counties: County Seat: Galesburg Named After: General Henry Knox
LAKE	Established: 1839 Parent Counties: McHenry County Seat: Waukegan Named After: From the fact that there are 40 small lakes within the county
LA SALLE	Established: 1831 Parent Counties: County Seat: Named After: La Salle, French explorer
LAWRENCE	Established: 1821 Parent Counties: Edwards County Seat: Lawrenceville Named After: Named for Captain James Lawrence, Commander of the Chesapeake, who was mortally wounded in an engagement between that vessel and the British vessel, Shannon, during the War of 1812.
LEE	Established: 1839 Parent Counties: Ogle County Seat: Dixon Named After: Richard Henry Lee (Rev. War Soldier) - Orator and statesman of the Revolutionary period, a member of the Continental Congress, a Representative in Congress and United States Senator from Virginia.
LIVINGSTON	Established: 1837 Parent Counties: County Seat: Pontiac Named After: Edward Livingston
LOGAN	Established: 1839 Parent Counties: Sangamon County Seat: Lincoln Named After: Dr. John Logan, father of Senator John A. Logan
MACON	Established: 1829 Parent Counties: County Seat: Named After: Nathaniel Macon, (Rev. War Soldier & statesman)
MACOUPIN	Established: 1829 Parent Counties: County Seat: Carlinville Named After: Native American word meaning "white potato"
MADISON	Established: 1812 Parent Counties: St.Clair County Seat: Edwardsville Named After: President James Madison (President at the time)
MARION	Established: 1823 Parent Counties: County Seat: Salem Named After: Francis Marion, soldier in revolution
MARSHALL	Established: 1839 Parent Counties: Putnam County Seat: Lacon Named After: Chief Justice John Marshall
MASON	Established: 1841 Parent Counties: Sangamon County Seat: Havana Named After: A county in Kentucky
MASSAC	Established: 1843 Parent Counties: County Seat: Metropolis Named After: Fort Massac
McDONOUGH	Established: Jan. 25, 1826, but attached to Schuyler until 1830 Parent Counties: Schuyler County Seat: Macomb Named After: Commodore McDonough
McHENRY	Established: 1836 Parent Counties: Attached to Lake county, split in 1839 County Seat: Woodstock Named After: General William McHenry
McLEAN	Established: 1830 Parent Counties: Sangamon County Seat: Bloomington Named After: John McClean, early U.S. Senator
MENARD	Established: 1839 Parent Counties: Sangamon County Seat: Petersburg Named After: Pierre Menard
MERCER	Established: 1825 Parent Counties: Warren County Seat: Aledo Named After: General Hugh Mercer
MONROE	Established: 1816 Parent Counties: County Seat: Waterloo Named After: President Monroe
MONTGOMERY	Established: 1821 Parent Counties: County Seat: Hillsboro Named After: General Richard Montgomery
MORGAN	Established: Jan. 31, 1823 Parent Counties: Sangamon (Parent of Cass and Scott counties) County Seat: Jacksonville Named After: General Daniel Morgan
MOULTRIE	Established: 1843 Parent Counties: Shelby & Macon County Seat: Sullivan Named After: Revolutionary War Hero --- Moultrie
OGLE	Established: 1837 Parent Counties: Jo Daviess County Seat: Oregon Named After: Lt. Joseph Ogle, politician
PEORIA	Established: 1825 Parent Counties: Fulton County Seat: Peoria Named After: Named after the Peoria Native American Tribe
PERRY	Established: 1827 Parent Counties: County Seat: Pinckneyville Named After: Com. Oliver H. Perry
PIATT	Established: 1841 Parent Counties: Macon & Dewitt County Seat: Monticello Named After: John Piatt's family, he was the first sheriff
PIKE	Established: 1821 Parent Counties: County Seat: Pittsfield Named After: Captain Zebulon Pike, explorer
POPE	Established: 1816 Parent Counties: Gallatin & Johnson County Seat: Golconda Named After: Judge Nathaniel Pope
PULASKI	Established: 1832 Parent Counties: Alexander County Seat: Mound City Named After: a Polish patriot who aided the Americans during the Revolutionary War
PUTNAM	Established: 1825 Parent Counties: Sangamon County Seat: Hennepin Named After: Gen. Israel Putnam
RANDOLPH	Established: 1795 Parent Counties: St. Clair County Seat: Chester Named After: Beverly Randolph
RICHLAND	Established: 1841 Parent Counties: Edwards County Seat: Olney Named After: A County in Ohio
ROCK ISLAND	Established: 1835 Parent Counties: County Seat: Rock Island (formerly known as Stephenson) Named After: An island in the Mississippi River
ST. CLAIR	Established: (1st county) April 27, 1790 Parent Counties: County Seat: Belleville Named After: Governor St. Clair
SALINE	Established: 1847 Parent Counties: County Seat: Harrisburg Named After: the salt springs in the county
SANGAMON	Established: June 30, 1821 Parent Counties: Bond & Madison County Seat: Springfield Named After: The Sangamon River, which flows through it
SCHUYLER	Established: 1825 Parent Counties: Pike County Seat: Rushville Named After: General Philip Schuyler
SCOTT	Established: 1839 Parent Counties: Morgan County Seat: Winchester Named After: Early pioneers in the county - SCOTT family
SHELBY	Established: 1827 Parent Counties: County Seat: Shelbyville Named After: Kentucky Governor Shelby
STARK	Established: 1839 Parent Counties: Putnam & Knox County Seat: Toulon Named After: John Stark, a soldier of the French and Indian wars, a Major General of the Revolution, who served with distinction at Bunker Hill, Trenton, Princeton, and Bennington.
STEPHENSON	Established: 1837 Parent Counties: County Seat: Freeport Named After: Benjamin Stephenson, prominent pioneer, a Colonel of the Territorial militia, and Adjutant General of the Territory (1813–1814).
TAZEWELL	Established: 1827 Parent Counties: Sangamon County Seat: Pekin Named After: Virginia Governor Tazewell
UNION	Established: January 1818 Parent Counties: County Seat: Jonesboro Named After: The Federal Union
VERMILION	Established: 1826 Parent Counties: County Seat: Danville Named After: Vermilion and Little Vermilion Rivers which flow through it
WABASH	Established: 1824 Parent Counties: Edwards County Seat: Mount Carmel Named After: The Wabash River, which flows through it
WARREN	Established: Created in 1825; Fully organized in 1830 Parent Counties: County Seat: Monmouth Named After: General Joseph Warren
WASHINGTON	Established: 1818 Parent Counties: One of the original 15 County Seat: Nashville Named After: President George Washington
WAYNE	Established: 1819 Parent Counties: County Seat: Fairfield Named After: General Anthony Wayne
WHITE	Established: 1816 Parent Counties: 10th county organized out of original 15 County Seat: Carmi Named After: Major Leonard White, killed in the battle of Tippecanoe
WHITESIDE	Established: 1836 Parent Counties: County Seat: Morrison Named After: Captain Samuel Whiteside, "noted Indian fighter"
WILL	Established: 1836 Parent Counties: Cook County Seat: Joliet Named After: Dr. Conrad Will, early politician and legislator
WILLIAMSON	Established: 1839 Parent Counties: Franklin County Seat: Marion Named After: A county in Tennessee
WINNEBAGO	Established: 1836 Parent Counties: LaSalle & Jo Daviess County Seat: Rockford Named After: The Winnebago Native American Tribe
WOODFORD	Established: 1841 Parent Counties: County Seat: Eureka Named After: A county in Kentucky

Sources:
State of Illinois, 175th Anniversary Blue Book, 1993-1994 (Counties A-J) Transcribed by Candi Horton
The Historical Enclyopedia, 1901 (Counties K-Z) Transcribed by Kim Torp