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.

It was formed out of LaSalle County and named after U.S. Attorney General Felix Grundy.

[4] The county was well known for its coal mines[5] and attracted miners from Pennsylvania and other regions to work its deposits.

[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.