 It was named for Henderson County, Kentucky, which was named for Richard Henderson, founder of the Transylvania Company, an early attempt to organize what later became Kentucky around 1775.According to the US Census Bureau, the county has a total area of , of which is land and (4.1%) is water.In recent years, average temperatures in the county seat of Oquawka have ranged from a low of in January to a high of in July, although a record low of was recorded in February 1996 and a record high of was recorded in August 1983.