The first settler was a Quaker named George Haworth, followed by James Martin, Abraham Hanline, Solomon Carter, and William Cordell.
Piatt County was formed in 1841 from Macon and Dewitt counties. Two local residents: James A. Piatt and Jesse Warner, were instrumental in forming the county. It was named after James A. Piatt after winning a coin flip against Jesse Warner.
Abraham Lincoln practiced law in Piatt County as a circuit lawyer. Abraham Lincoln and Stephen A. Douglas planned their presidential debates in Piatt County in 1858. One of which is ornamented by a marker just south of Monticello.
The first courthouse was built in 1843 but replaced by the current courthouse which was finished in 1904.
Illinois Power Company was a major electric utility in Central Illinois, centered in Decatur, Illinois, to the west of Piatt County. At one time, Illinois had a "personal property tax", an ad valorem tax levied by the counties on property that was not real estate. The personal property tax was a major expense for the electric utilities, since their generators and transmission lines were "personal property". Under Illinois law, a corporation, such as Illinois Power, paid personal property tax to the county in which the corporate headquarters was located. Because Piatt County offered a low tax rate, Illinois Power moved its corporate headquarters to that county. This allowed Piatt County to tax utility assets over half of the State, providing a rich source of revenue which was responsible for much of the wealth of this tiny county.