Winfield is a city situated along the Walnut River in the west-central part of Cowley County, located in South Central Kansas, in the Central United States. The population was estimated to be 11,861 in the year 2005. It is the county seat and most populous city of Cowley County. Winfield is home to Southwestern College, a four-year private Methodist college and once also held St. John's College before it closed in 1986. The city is well known for hosting the Walnut Valley Festival, a large bluegrass and acoustic music festival, on the third weekend of every September and the Winfield Country Roundup in the first weekend in June. The fictional castaway Mary Ann Summers from Gilligan's Island hailed from Winfield.