The first settler was George Haworth, a Quaker, 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. 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. It was replaced by the current courthouse in 1904. Illinois Power Company was a major electric utility in Central Illinois, centered in Decatur, 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.
Geography
According to the US Census Bureau, the county has a total area of, of which is land and is water.
Climate and weather
In recent years, average temperatures in the county seat of Monticello have ranged from a low of in January to a high of in July, although a record low of was recorded in January 1999 and a record high of was recorded in July 1966. Average monthly precipitation ranged from in January to in August.
As of the 2010 United States Census, there were 16,729 people, 6,782 households, and 4,823 families residing in the county. The population density was. There were 7,269 housing units at an average density of. The racial makeup of the county was 98.0% white, 0.3% black or African American, 0.3% Asian, 0.2% American Indian, 0.2% from other races, and 0.9% from two or more races. Those of Hispanic or Latino origin made up 1.0% of the population. In terms of ancestry, 26.1% were German, 21.9% were American, 15.2% were English, and 13.4% were Irish. Of the 6,782 households, 32.0% had children under the age of 18 living with them, 59.4% were married couplesliving together, 8.0% had a female householder with no husband present, 28.9% were non-families, and 24.5% of all households were made up of individuals. The average household size was 2.46 and the average family size was 2.92. The median age was 42.6 years. The median income for a household in the county was $55,752 and the median income for a family was $65,850. Males had a median income of $50,425 versus $32,304 for females. The per capita income for the county was $26,492. About 5.1% of families and 7.6% of the population were below the poverty line, including 10.8% of those under age 18 and 4.5% of those age 65 or over.
