The first settler was George Haworth, a Quaker, followed by James Martin, Abraham Hanline, Solomon Carter and William Cordell.

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.