McDowell County, the southernmost county in West Virginia, was created on February 28, 1858, from part of Tazewell County, Virginia. The new county was named after James McDowell, a governor of Virginia (1843—45). It consists of 535 square miles of rugged mountain land, on the headwaters of the Tug Fork. McDowell County is served by U.S. 52, several state and county routes, and the Norfolk Southern Railroad. The county seat is Welch.

After the Revolutionary War, the federal government granted vast tracts of unoccupied land to military veterans and others. Many of these grants passed into the hands of speculators. In 1795, the original owners of all the land now included in McDowell County, Wilson Nicholas and Jacob Kenney, sold their grant to Robert Morris, a Philadelphia financier and a signer of the Declaration of Independence. Morris could not effectively control such a large acreage. Settlers began moving in, and Morris lost control of his lands as the newcomers asserted squatters' rights. By the 1830s, much acreage had fallen back into public ownership in tax forfeitures. On the eve of the Civil War, the state of Virginia owned two-thirds of the land in McDowell County.

The first white residents of present McDowell County were Mathias and Lydia Harman, who settled in a cabin along the Dry Fork about 1802. In 1829, William Fletcher received 20 acres of land at the site of present Welch. Settlement was slow, due to the rugged terrain. In 1860, there was a population of only 1,535 in the new county.

Apart from sporadic guerrilla warfare, the Civil War largely passed by McDowell County. Nonetheless, McDowell had been founded on the eve of war, and the unsettled conditions hindered the orderly commencement of county affairs. The notion of the "free State of McDowell," later a motto expressing the county's independent ways, originated during this free-wheeling period.

For years, citizens battled over the site of the county seat. Between 1858 and 1872, the county court met at various places. In 1872, the county seat was located at Perryville (now English), the largest town at that time. Two decades later, the county seat controversy arose again. The population around Welch had increased significantly with the arrival of the Norfolk & Western Railway in the early 1890s, and the town was incorporated in 1894. McDowell Countians voted to move the county seat to Welch in 1892, amid allegations of fraud. Violence seemed imminent, but James A. Strother and Trigg Tabor secretly moved the county records to Welch, where the county seat has remained to this day.

Until the late 19th century, the county's vast coal deposits remained largely untouched. By the 1870s, investors began purchasing much of McDowell County in anticipation of the arrival of the railroad, which would make it possible to ship coal and timber to market. By 1892, the Norfolk & Western Railway had spanned the county and within the next decade constructed spurs into most of the coal mining areas.

The native population was not large enough to supply the labor needs of the new mines. As coal companies recruited immigrants from southern and eastern Europe and blacks from the American South, McDowell became one of the most diverse counties in the state and remained so for many years.

McDowell, which had no slave population and no free blacks after emancipation, became the state's center of African-American population in the industrial era. McDowell County blacks established a power base within the state and local Republican Party, governing communities such as Keystone in the early 20th century and regularly sending delegates to the state legislature. A fourth of the population was black in 1950. In 2009, African-Americans made up 10.9 percent of the county's population.

By the mid-20th century, McDowell was the leading coal producing county in the nation, until it was surpassed by neighboring Logan County in 1955. McDowell established itself as the third most populous county in the state in 1920, and its population peaked at 98,887 in 1950. The county had an estimated 21,326 residents in 2012.

After 1950, mine mechanization led to widespread job losses, and people left to seek work elsewhere. The loss of coal jobs and the lack of economic diversification brought hard times to McDowell County. By the end of the 20th century, McDowell was the poorest county in West Virginia. In 1999, 47.5 percent of McDowell County families with children lived in poverty. The primary employers include numerous small coal mines, the state of West Virginia, and the county school board.

McDowell County has two public high schools, Mount View and River View. Mount View High School, located at Welch, is also used as an off-campus site for Bluefield State College. River View High School, located at Bradshaw, opened in the fall of 2010 and consolidated the former Iaeger and Big Creek high schools. In 2001, the state Board of Education took over administration of the McDowell County school system, declaring a state of emergency and appointing a new county superintendent of schools. In 2013 the state board voted to return control of McDowell County schools to local authorities. The state cited progress credited in part to Reconnecting McDowell, a public-private collaboration of state and national organizations working since 2011 to improve conditions in McDowell County.

McDowell County has ten incorporated communities: Welch, Kimball, Keystone, Northfork, Gary, Anawalt, Davy, Iaeger, War, and Bradshaw. Most residents live outside these municipalities, however, usually in former coal company towns. There are two wildlife management areas, Berwind Lake and Anawalt, as well as Panther State Forest. The county's only golf course is at the Gary Country Club.

The primary river is the Tug Fork, a Big Sandy tributary which flows through the northern section of the county. The major tributaries of the Tug Fork in McDowell County are Elkhorn Creek and Dry Fork. McDowell County suffered severe flooding in 2001 and 2002. On July 8, 2001, the Tug reached 19.77 feet, more than two feet above flood stage. On May 2, 2002, the river crested at a record 22.10 feet, more than five feet above flood stage. The floods destroyed or damaging hundreds of homes and businesses. The 2002 floods and related weather killed four people.

The McDowell County coalfields gained national attention with the publication of Coalwood native Homer Hickam's 1998 book, Rocket Boys, and the subsequent film, October Sky. The first World War I memorial was built at Welch, and the nation's first World War I memorial for African-American veterans was located at Kimball. Fire gutted both buildings, with the Welch memorial being destroyed. The Kimball War Memorial, which was placed on the National Register of Historic Places in 1993, has been extensively renovated and is once again an active community center. Welch is also the location of the first municipal parking building in the nation.

McDowell County has the distinction of having had the first female African-American state legislator in the United States, when Minnie Buckingham Harper was appointed to the House of Delegates by Governor Gore in 1928. Other important natives include Henry D. Hatfield, governor of West Virginia (1913—17) and U.S. senator (1929—35); state Attorney General Edward P. Rucker (1897—1901); and state Attorney General Ira J. Partlow (1945—49).