Longford is a city in Clay County, Kansas, United States. As of the 2020 census, the population of the city was 73. Longford was founded in 1…Longford is a city in Clay County, Kansas, United States. As of the 2020 census, the population of the city was 73. Longford was founded in 1870. The first post office in Longford was established in 1875.