In 1822 a tract of land which included the area now known as Plainfield was obtained by Jeremiah Hadley of Preble County, Ohio. Ten years later he sold it to his son, Elias Hadley. Levi Jessup and Elias Hadley laid out the town in 1839. Plainfield was incorporated as a town in 1839. The town got its name from the early Friends (Quakers) who settled around the area and established several meetinghouses throughout the county, including the important Western Yearly Meeting of Friends in Plainfield. The Friends were "plain" people, and thus the name "Plainfield". The high school continues to honor the Quakers, using the name for the school's mascot.