Marshall County is situated at the base of the Northern Panhandle. It is bordered by Ohio County to the north, Wetzel County to the south, Pennsylvania to the east, and the Ohio River to the west. The county seat is Moundsville. Marshall was created from part of Ohio County on March 12, 1835. The county was named for John Marshall, chief justice of the U.S. Supreme Court (1801-35).

The early settlers were astonished by the imposing size of the Grave Creek Mound, at 62 feet high and 240 feet in diameter the largest of the conical mounds built by the prehistoric Adena people. In 1771, Joseph Tomlinson, the original European owner of the mound, established the first settlement on the site that became Moundsville. Following the mound's excavation in 1838, its first museum opened in 1839. The state acquired the mound in 1909 and opened a museum in 1952, which was replaced by the Delf Norona Museum in 1978.

Christopher Gist was the first European to record a visit to present Marshall County, while exploring for the Ohio Company in 1751. The first settlers of European descent were John Wetzel and his family in 1769 or 1770. They were soon followed by the Zanes and others. During the Revolutionary War, settlers depended upon Fort Henry, erected at Wheeling in 1774, for protection against the Indians and their British allies.

The Western Virginian and People's Press, established in Elizabethtown in 1831, was the first of at least 26 newspapers. The Moundsville Daily Echo, established as a weekly in 1891 and expanded to daily publication in 1896. The last newspaper in Marshall County, it continued to be published until shutting down in 2024.

The Ohio River was the chief transportation route until the completion of the Baltimore & Ohio Railroad from Baltimore to Wheeling at Rosby's Rock near Moundsville, December 24, 1852. In the 1890s, Benwood, McMechen, and Moundsville were connected by an electric railway that provided trolley cars until replaced by buses in 1941. State Route 2 became an important north-south highway with its completion in 1936. U.S. 250, State Routes 86 and 88, and numerous county routes have long provided a network of roads for Marshall County.

The State Penitentiary was established in Moundsville in 1866. Both men and women were inmates until a women's prison opened in Summers County in 1948. Eighty-five men were hanged and 19 electrocuted at Moundsville between 1899 and the abolition of capital punishment in 1965. The penitentiary was closed in 1995 and has been transformed into a tourist attraction.

Marshall County's economy was agricultural through most of the 19th century, and agriculture remains important today. Industrial activity began in Benwood with ironmaking in the 1850s. Steelmaking surpassed iron, and the Benwood works grew into large steel mills by 1900. The production of lumber, coal, coke, limestone, and clay developed into important Benwood-area industries. Moundsville's early industries included brooms, buggy whips, bricks, building supplies, and glassware. The Fostoria Glass Company opened in Moundsville in 1892 and produced high-quality tableware. The United States Stamping Company, producers of enameled cooking ware, began operations in 1901.

In the early 20th century, industrial activity expanded into paper box manufacturing, pottery, well-drilling tools, women's clothing, guns, and various building materials. Wheeling Metal and Manufacturing Company opened in Glen Dale in 1904 to produce metal ceilings, metal roofs, gutters, and downspouts. The Leadclad Wire Company was established in 1922 by Wheeling Metal to produce rustproof fencing. The United Zinc Smelting Corporation operated from 1918 to 1945. In the late 1920s, the arrival of the Fokker Aircraft Corporation, the Triangle Conduit Company, and the Judelson Dryer Company created hundreds of new jobs. An explosion on April 28, 1924, at Benwood's Wheeling Steel coal mine killed an entire shift of 119 men. Louis Marx & Company opened the world's largest toy plant in 1934 at the former Fokker Aircraft plant in Glen Dale, employing 800 workers in the 1970s. The toy factory closed in 1980.

The chemical industry gained importance during World War II with the opening of major plants, including Allied Chemical, Pittsburgh Plate Glass, and Mobay. Industrial expansion in the 1950s and 1960s included coal mining, electric power generation, carbon, glass, tools, primary metals, and bronze casting. In the last decades of the 20th century, industry declined. Fostoria Glass closed in 1986. Coal mining remains important. In 2022, Marshall County led the state in coal pdocution, generating 16 million tons, nearly one-fifth of West Virginia’s entire coal output, and employed 12 percent of the state’s mine workforce.

The primary churches in the 19th century were Methodist, Presbyterian, and Episcopal, and many Protestant and Catholic churches are represented in the county today. Reynolds Memorial Hospital, founded by B. M. Spurr in 1899 to care for discharged convicts and the unfortunate, opened a new facility in Glen Dale in 1963. The Hare Krishna community of New Vrindaban constructed the Palace of Gold beginning in 1973, and it became an important tourist attraction.

Marshall County's incorporated communities are Benwood, Cameron, Glen Dale, McMechen, and Moundsville. In 2020, Marshall County's population was 30,591, down from its peak of 41,608 in 1980. The county has an area of 311.8 square miles.

In 2023, Marshall County's largest employers were the county school system, Marshall County Coal Resources (a subsidiary of American Consolidated Natural Resources), Reynolds Memorial Hospital, Pleasant Valley Hospital, and Westlake Management Services.

Notables from Marshall County include Delf Norona, archeologist, historian, and philatelist; Davis Grubb, novelist and short-story writer; Harriet B. Jones, the first female doctor in West Virginia; Arch Moore, three-time governor of West Virginia; Edwin Holt Hughes, Methodist bishop who was instrumental in uniting the three branches of Methodism in 1939; C. B. Allman, the biographer of Lewis Wetzel, an early county resident and Indian fighter; and country music superstar Brad Paisley.