Grundy County is a county in the U.S. state of Illinois. According to the 2020 census, it has a population of 52,533.^[1] Its county seat is Morris.^[2]

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Grundy County
County
Grundy County Courthouse
Flag Seal
Location within the U.S. state of Illinois
Illinois's location within the U.S.
Coordinates: 41°17′N 88°26′W
Country	United States
State	Illinois
Founded	February 17, 1841
Named for	Felix Grundy
Seat	Morris
Largest city	Morris
Area
• Total	430 sq mi (1,100 km2)
• Land	418 sq mi (1,080 km2)
• Water	12 sq mi (30 km2)  2.9%
Population  (2020)
• Total	52,533
• Density	120/sq mi (47/km2)
Time zone	UTC−6 (Central)
• Summer (DST)	UTC−5 (CDT)
Congressional district	16th
Website	www.grundyco.org

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Grundy County is part of the Chicago-Naperville-Elgin, IL-IN-WI Metropolitan Statistical Area.

In 2010, the center of population of Illinois was in Grundy County, just northeast of the village of Mazon.^[3]

Illinois's state fossil, the unique and bizarre Tully Monster, was first found in Mazon Creek. Grundy County is home to Dresden Generating Station—the first privately financed nuclear power plant built in the United States—and the Morris Operation—the only de facto high-level radioactive waste storage site in the United States.

Grundy County was established on February 17, 1841. It was formed out of LaSalle County and named after U.S. Attorney General Felix Grundy (1777–1840).^[4] The county was well known for its coal mines^[5] and attracted miners from Pennsylvania and other regions to work its deposits. The 1883 Diamond Mine Disaster occurred in Grundy County. The disaster took the lives of 69 men and boys who were trapped underground when water broke through into the mine after days of heavy rain and the pumps could not keep up with the rising water. 22 bodies were eventually recovered, the remaining 44 were left in the mine and the mine was sealed. Today a marker stands near where it was believed the majority of victims were entombed.^[6]

• Grundy County at the time of its creation in 1841

According to the U.S. Census Bureau, the county has a total area of 430 square miles (1,100 km²), of which 418 square miles (1,080 km²) is land and 12 square miles (31 km²) (2.9%) is water.^[7]

As of the 2010 United States Census, there were 50,063 people, 18,546 households, and 13,431 families residing in the county.^[15] The population density was 119.8 inhabitants per square mile (46.3/km²). There were 19,996 housing units at an average density of 47.8 per square mile (18.5/km²).^[7] The racial makeup of the county was 93.7% white, 1.2% black or African American, 0.7% Asian, 0.2% American Indian, 2.7% from other races, and 1.5% from two or more races. Those of Hispanic or Latino origin made up 8.2% of the population.^[15] In terms of ancestry, 28.3% were German, 23.0% were Irish, 12.7% were Italian, 9.4% were Polish, 8.6% were English, 7.1% were Norwegian, and 3.0% were American.^[16]

Of the 18,546 households, 38.6% had children under the age of 18 living with them, 57.8% were married couples living together, 9.7% had a female householder with no husband present, 27.6% were non-families, and 22.5% of all households were made up of individuals. The average household size was 2.69 and the average family size was 3.16. The median age was 36.1 years.^[15]

The median income for a household in the county was $64,297 and the median income for a family was $75,000. Males had a median income of $58,491 versus $36,592 for females. The per capita income for the county was $27,895. About 5.2% of families and 6.9% of the population were below the poverty line, including 8.0% of those under age 18 and 6.5% of those age 65 or over.^[17]

• National Register of Historic Places listings in Grundy County, Illinois