Clarksville is a city in Butler County, Iowa, United States, along the Shell Rock River. The population was 1,264 at the 2020 census. A post offi…Clarksville is a city in Butler County, Iowa, United States, along the Shell Rock River. The population was 1,264 at the 2020 census. A post office opened in Clarksville in 1861. Clarksville is located at 42°46'58" North, 92°40'11" West.