Hardin is a city in and the county seat of Big Horn County, Montana, United States. The population was 3,818 at the 2020 census. It is loc…Hardin is a city in and the county seat of Big Horn County, Montana, United States. The population was 3,818 at the 2020 census. It is located just north of the Crow Indian Reservation. The city was named for Samuel Hardin, a friend of developer Charles Henry Morrill. It was incorporated in 1911. Thomas D. Campbell operated the nation's largest wheat farm on 95,000 acres near here and pioneered the use of industrial machinery in farming. The Holly Sugar Company established a sugar beet–processing plant in Hardin in 1937.