Its county seat is Benton. The county is named for John Marshall, Chief Justice of the United States from 1801 to 1835. Until 2004, it was a prohibition or dry county, when residents of Calvert City voted to allow sales of liquor by the drink in restaurants. It is the only Purchase Region county to not border another state.

The county was formed in 1842 from part of Calloway County. The first settlement was around 1818, when the area was bought from Native Americans as part of the Jackson Purchase.

From its settlement until the 1930s, the county was nearly completely agricultural. The creation of Kentucky Lake by the Tennessee Valley Authority in the 1940s brought tourism and industry to the county with resorts along the lake and chemical and manufacturing plants, mostly in the Calvert City area, attracted by the dam's cheap and plentiful electricity. The creation of the lake led to the destruction of two Marshall County towns, Birmingham, about six miles north of the present hamlet of Fairdealing, and Gilbertsville, which was at the present dam site. Gilbertsville was rebuilt somewhat to the west of its original location. Birmingham residents were dispersed. Gilbertsville was an incorporated town until the 1970s, when its charter was dissolved by public vote.