Iroquois County was formed in 1833 out of Vermilion County. It was named for the Iroquois people. The county seat is located a few miles east of the center of the county in Watseka, Illinois. Iroquois County is the third largest county in Illinois, being 35 miles long and 32 miles wide, containing 1,120 square miles. Iroquois County is bounded on the north by the county of Kankakee, on the east by the State of Indiana, on the south by Vermilion and Ford Counties, and on the west by Ford County. 
The county seat is  Watseka.