Illinois Counties
Illinois County map
Click Image to Enlarge

Illinois Counties

There are one hundred and two counties in Illinois. St. Clair County was the first county established in what is today Illinois. Most counties in Illinois were named after early American leaders, especially of the American Revolutionary War, as well as soldiers from the Battle of Tippecanoe and the War of 1812.

Mercer County, Illinois

Mercer County Education, Geography, and HistoryMercer County, Illinois Courthouse

Mercer County is a county located in the state of Illinois. According to the 2010 census, it had a population of 16,434. Mercer County was created on January 13, 1825.  The county seat is Aledo. The county is named for General Hugh Mercer (1726-1777), British officer in the Seven Years' War and general in the Continental Army during the American Revolutionary War.

Mercer County is included in the Davenport-Moline-Rock Island, IA-IL Metropolitan Statistical Area.

Etymology - Origin of Mercer County Name

The county is named for General Hugh Mercer, killed at the Battle of Princeton during the American Revolution.


County QuickFacts: CensusBureau Quick Facts

Mercer County History

Mercer County was created on January 13, 1825 (Laws, 1825, p. 93) and was formed from unorganized land (Pike County). County organization was completed in 1835. Present area, or parts of it, formerly included in: Attached to Warren County [Laws, 1831, p. 68.] (1831-1835), Attached to Peoria County [Laws, 1826, p. 76.] (1826-1831), Attached to Schuyler County [Laws, 1825, p. 95.] (1825-1826), Pike County (1821-1825), Madison County (1812-1821) and St. Clair County (1795-1812).

The County was named for Hugh Mercer, a General of the Revolution, killed at the battle of Princeton. The County Seat is Aledo . Prior County Seats was New Boston- Temporary designation (1835-1837), Millersburg (1837-1847), Keithsburg (1847-1857) and Aledo (1857-Present).

Geography: Land and Water

As reported by the Census Bureau, the county has a total area of 569 square miles (1,473 km2), of which, 561 square miles (1,453 km2) of it is land and 8 square miles (20 km2) of it (1.38%) is water.

Mercer county is located in northwest Illinois.

Neighboring Counties

Bordering counties are as follows:

  • Northeast: Rock Island County; Henry County
  • Southeast: Knox County; Warren County
  • Southwest: Henderson County; Des Moines County, Iowa
  • West: Louisa County, Iowa


Compare More Colleges and Universities
Find the Right School

Find more schools to match to your needs.

County Resources
US Counties
Click Image to Enlarge