By action of the General Assembly of Illinois, the County of McLean, Illinois was formed on December 25, 1830, out of Tazewell County. It was named for John McLean, the first Representative of Illinois in Congress (1818) and United States Senator (1824-25) for Illinois, who died in 1830.

 McLean County has the largest land area of any county in the in the State of Illinois, encompassing approximately 1,186 square miles of which 1,183 are land. McLean County is larger than the land area of the State of Rhode Island.  The City of Bloomington is the seat of County Government.