The 1854 Gazetteer of the United States by Thomas Baldwin shows:

PIATT, a county in the E. central part of Illinois, has an area of 270 square miles. It is intersected by the North fork of Sangamon river, which flows in a S. W. direction. The surface is nearly level, and the soil fertile. The county consists partly of prairie and partly of timbered land, the former being the more extensive. Indian corn, wheat, oats, and pork are the staples. In 1850 the county produced 430,655 bushels of corn; 5769 of wheat; 29,115 of oats, and 37,950 pounds of butter. It contained 2 churches, and 300 pupils attending public schools. Named probably in honor of Colonel Piatt, a Western pioneer. Capital, Monticello. Pop., 1606.