Fairfield is a city in, and the county seat of, Jefferson County, Iowa, United States. It has a population totaling 9,464 people according to the 2010 census. It is a Midwestern city surrounded by rolling farmlands filled with corn, soybean, cattle, and hogs with a median family income of $46,138 (10% of families below the poverty line). The city became the county seat in 1839 with 110 residents and grew to 650 by 1847. Its library was established in 1853, and it held its first fair in 1854. Early architecture includes work by George Franklin Barber and Barry Byrne, who trained under Frank Lloyd Wright.