Clay County is located in central West Virginia, northeast of Charleston. It occupies 346.4 square miles. The Elk River bisects the county from east to west, entering at Duck and leaving at King Shoals. The town of Clay is the county seat.

Rugged, laurel-covered hollows dart back from the narrow river valley, and level land is at a premium. Early settlers did not find the area particularly inviting, so the population was sparse during the 18th and 19th centuries. Yet the soil was rich, and some beautiful hillside farms developed during the days when Western Virginia was being settled and later, after West Virginia had become a state.

The first known person of European descent to occupy the area was Sinnett Triplett, who came about 1812. He had a camp on what is now called Sinnett Branch, a tributary of Lilly Fork of Buffalo Creek, a few miles south of the present town of Clay. Triplett was followed by David McOlgin, who settled on the north side of Elk River near where the town of Clay would be located. McOlgin is considered the first permanent settler. By 1839, 28 families lived in what is now Clay County. The early settlers survived by farming, hunting, and later by logging. It was common to run log rafts down the Elk River, sell the logs in Charleston, then walk home.

Clay County was created on March 29, 1858, from parts of Nicholas and Braxton counties and named for Henry Clay, the great U.S. senator from Kentucky. Clay was popular among the residents of Western Virginia because he often supported their causes. Two years after its formation, the 1860 census showed 1,787 people living in the county. By 1890 the figure had increased to 4,659.

There was internal strife in Clay County during the Civil War. Several Southern sympathizers were carted off to the military prison at Camp Chase, Ohio, during the conflict. It is believed that county resident Sol Carpenter was murdered because of his Southern leanings. Other Clay County men joined the Union Army.

The coming of the railroads during the 1890s brought substantial increases in both population and prosperity. The county had an abundance of coal and timber, and large corporations began operating in the area. Coal mining started out small, with Clay County producing only 2,860 tons in 1904, but eventually became a major industry. Much of the coal production was centered in Widen in the northeast corner of the county, a thriving private town controlled by industrialist Joseph G. Bradley. Known locally as "J. G.," Bradley was for many years the major power in Clay County. By 1960, the county was producing 900,000 tons of coal per year. Clay County’s deep mine production dwindled by 2001. In 2009 deep mining had rebounded and accounted for approximately 500,000 tons of the three million tons produced in the county. However, in 2021 and 2022, Clay County reported no coal mined despite two of its largest employers being coal companies.

At one time Clay County was served by the Buffalo Creek & Gauley Railroad, which connected the coalfields at Widen to the Baltimore & Ohio Railroad at Dundon, near the town of Clay. The B&O then carried Clay County coal to the national market. Currently, there is no regular rail service.

The Interstate highway system, constructed during the last half of the 20th century, nearly missed Clay County. I-79 barely notches the northern tier of the county as it makes its way from Morgantown to Charleston. The county is served by State Route 4 which, for the most part, follows the Elk River to Charleston; State Route 16 from Gauley Bridge to Big Otter; and State Route 36, which intersects State Route 4 near Maysel and leads the traveler to I-79. Modern highway connections have made Charleston more accessible, and many Clay Countians have found employment in the Kanawha Valley.

Clay is one of West Virginia's poorest counties, with a fourth of its people living in poverty at the turn of the 21st century. Yet the county has an excellent system of public schools. Clay County is served by one high school, one middle school, and four elementary schools. Both Clay County High School and Clay Middle School have been named National Schools of Excellence. One graduate, Carolyn Conner, distinguished herself at West Virginia University and went on to become a Rhodes Scholar.

The population of Clay County peaked in 1940 at 15,206, but the loss of mining jobs during the 1950s and '60s caused that number to drop to 9,330 by 1970. Widen was the scene of a fatal labor confrontation in 1952, and the company sold its mining operation in 1959. The once-thriving town all but disappeared during the 1960s. There were slight ups and downs in the county population from 1970 to 2000. During the 21st century, Clay County’s population declined from 10,330 in 2000 to 8,051 in 2020.

In 2016, a deadly flood along the Elk River devastated the county seat of Clay and temporarily cut off nearby communities. Since the 2023 closing of a market in Clay, the county has lacked a market where residents can purchase fresh produce.

As of 2023, Clay County's largest employers were the county school system, Consol Mining, Clay Health Care Center, Clay Senior and Community Services, and Screaming Eagle Coal.

Clay County has always been proud of its mountain music. Many of its old-time musicians have distinguished themselves at state and national festivals. French Carpenter, Doc White, Ira Mullins, Lee Triplett, Wilson Douglas, and Jenes Cottrell were all well known in their time as traditional mountain musicians. That tradition continues as John Morris of Ivydale and others have become staple performers at West Virginia music festivals.

The rugged hills of Clay County have produced some distinguished Americans. Lloyd H. Elliott left Clay County, attended Glenville State College, and went on to become president of both the University of Maine and George Washington University, and later, president of the National Geographic Society. Another Clay Countian, Neil Boggs, became a nationally known news commentator. Clay County made a contribution to world horticulture when the Golden Delicious apple was discovered on the Mullins farm near the head of Porter Creek in 1912.