Prestonsburg is a small home rule-class city in and the county seat of Floyd County, Kentucky, United States. It is in the eastern part of the …Prestonsburg is a small home rule-class city in and the county seat of Floyd County, Kentucky, United States. It is in the eastern part of the state in the valley of the Big Sandy River. The population was 3,255 at the time of the 2010 census, down from 3,612 at the 2000 census.