Will County is located in the northeastern part of the state of Illinois. According to the 2020 census, it had a population of 696,355, an increase of 2.8% from 677,560 in 2010, making it Illinois's fourth-most populous county. The county seat is Joliet. Will County is one of the five collar counties of the Chicago metropolitan area. The portion of Will County around Joliet uses area codes 815 and 779, while 630 and 331 are for far northern Will County and 708 is for central and eastern Will County.

Will County was formed on January 12, 1836, out of Cook and Iroquois Counties. It was named after Conrad Will, a politician and businessman involved in salt production in southern Illinois. Will was a member of the first Illinois Constitutional Convention and a member of the Illinois legislature until his death in 1835. The county originally included the part of Kankakee County, Illinois, north of the Kankakee River. It lost that area when Kankakee County was organized in 1852. Since then its boundaries have not changed.

36 locations in Will County are on the National Register of Historic Places.

"WILL, a county in the E. N. E. part of Illinois, bordering on Indiana, has an area of 1,236 square miles (3,200 km²). It is intersected by the Kankakee and Des Plaines Rivers, branches of the Illinois. The surface is generally level, and destitute of timber, excepting small groves. The soil is very fertile, and much of it is under cultivation. The soil of the prairies is a deep, sandy loam, adapted to Indian corn and grass. In 1850 the county produced 527,903 bushels of Indian corn; 230,885 of wheat; 334,360 of oats; 32,043 tons of hay, and 319,054 pounds of butter. It contained 14 churches, 3 newspaper offices; 3472 pupils attending public schools, and 200 attending other schools. Quarries of building stone are worked near the county seat. The Des Plaines river furnishes water-power. The county is intersected by the Illinois and Michigan canal, by the Chicago branch of the Central railroad, the Chicago and Mississippi, and by the Chicago and Rock Island railroad. Named in honor of Conrad Will, for many years a member of the Illinois legislature. Capital, Joliet. Population 16,703."

According to the U.S. Census Bureau, the county has an area of 849 square miles (2,200 km²), of which 837 square miles (2,170 km²) is land and 12 square miles (31 km²) (1.5%) is water.

The Kankakee River, Du Page River and the Des Plaines River run through the county and join on its western border. The Illinois and Michigan Canal and the Chicago Sanitary and Ship Canal run through Will County.

A number of areas are preserved as parks (over 20,000 acres (81 km²) total) under the Forest Preserve District of Will County. The 17,000 acres (69 km²) Midewin National Tallgrass Prairie is a U.S. Forest Service park in the county on the grounds of the former Joliet Arsenal. Other parks include Channahon State Park and the Des Plaines Fish and Wildlife Area.

In recent years, average temperatures in the county seat of Joliet have ranged from a low of 13 °F (−11 °C) in January to a high of 85 °F (29 °C) in July, although a record low of −26 °F (−32 °C) was recorded in January 1985 and a record high of 104 °F (40 °C) was recorded in June 1988. Average monthly precipitation ranged from 1.58 inches (40 mm) in January to 4.34 inches (110 mm) in July.