Flags fly off Louisiana State Highway 15 in Winnsboro (May 2013). Winnsboro is a city in and the parish seat of Franklin Parish, Louisiana, United States. As of the 2010 census, the population was 4,910, down from 5,344 at the 2000 census. The city is 59 percent African American. U.S. Highway 425 passes north–south through Winnsboro concurrent with Louisiana Highway 15 and extends northward to Rayville, the seat of neighboring Richland Parish.