Named for Wilson Cary Nicholas (1761-1820), a Virginia governor and U.S. senator, Nicholas County was created by the Virginia legislature from parts of Greenbrier, Kanawha, and Randolph counties on January 30, 1818. Located in central West Virginia on the Allegheny Plateau, Nicholas County encompasses 654.1 square miles, with economic resources including bituminous coal, limestone quarries, timber, fruit farms, tobacco, and livestock. Its two largest cities are Summersville, the county seat, and Richwood. In 2020, the county population was 24,604, down slightly from its peak of 28,126 in 1980.

In 1775, Maj. William Morris, accompanied by an enslaved Peter Morris, claimed lands in present Nicholas County and offered them to his oldest son, William Jr., who sold them to his brother, Henry. Henry moved to the land in 1791 and built a cabin for his family along Peters Creek, named for his father's slave. Other settlers followed. In 1792, two of the Morris daughters were killed by the renegade Simon Girty, who had stayed with the family under false pretenses during the winter. On April 7, 1818, the first meeting of the Nicholas County court was held at the home near Keslers Cross Lanes of John Hamilton, who donated 30 acres to establish the county seat, named Summersville in 1820 and incorporated in 1860.

Nicholas County was bisected by the Weston & Gauley Bridge Turnpike, completed in 1859. This north-south thoroughfare, connecting Gauley Bridge and Weston via Summersville, contributed to the development of the county.

Nicholas County was the site of military encounters in 1861 that proved to be an early turning point in the Civil War. On August 26, Gen. John Floyd led his Confederate forces across the Gauley River to attack the Union's 7th Ohio Regiment at Keslers Cross Lanes. The assault took the Union forces by surprise, and they were routed. On September 10, additional Union forces advanced against Floyd at Carnifex Ferry, prompting the Confederates to retreat and keeping the strategic Kanawha Valley under Union control for the early part of the war. In October, Robert E. Lee, who had been assigned to Western Virginia, decided not to attack Union forces under Gen. William S. Rosecrans.

In July 1861, Confederate spy Nancy Hart led an attack that burned most of Summersville to the ground. She was later captured and jailed but used her beauty to lure a guard into giving her his pistol, whereupon she shot him dead and escaped. After the war, she returned to neighboring Greenbrier County, married, and lived there until her death.

Around the turn of the 20th century, the railroad brought economic growth to Nicholas County. The lumber industry expanded rapidly, and the Cherry River Boom and Lumber Company built one of the world's largest sawmills at Richwood. Other industries followed, including a paper mill, handle and hub factories, a tannery, and what was then the world's largest clothespin factory. Later in the 20th century, coal led another economic surge, until the coal industry softened in the 1970s and early 1980s. More recently, Nicholas County has sought economic diversification. Along with Fayette, Raleigh, and Summers counties, Nicholas County is part of the 4C Economic Development Authority, which was created to foster regional economic growth. The completion of four-lane Appalachian Corridor L (U.S. 19) through Nicholas County in the 1990s brought heavy north-south traffic and prosperity to roadside businesses.

Nicholas Countians work in manufacturing, commerce, and for the service industries and government. Some work as miners and a few hundred on farms. In 2023, the largest employers were the county school system, WVU Medicine, Walmart, Columbia West Virginia, and Contura Energy.

Nicholas County has 15 schools with nearly 3,500 students. In 1914, the first Nicholas County High School opened in Summersville. The Renaissance Revival building was designed by noted Charleston architect H. Rus Warne. From 1915 to 1930, it also served as a normal school for training teachers. The high school remained open until a new one was built in 1978. The original building, listed in the National Register of Historic Places, is now the Old Main Arts & Heritage Center.

County elected officials include three county commissioners, a county clerk, a sheriff, a prosecuting attorney, an assessor, and a circuit clerk. Summersville and Richwood have municipal police departments. The judiciary in the county includes two magistrates, a county court judge, a municipal police court judge, and a state circuit court judge.

Summersville Lake, the largest impoundment lake in West Virginia, was dedicated by President Lyndon B. Johnson on September 3, 1966. Its 60 miles of shoreline and 2,800-acre surface offer recreational benefits in addition to the flood control and other purposes for which it was created. The lowering of the reservoir each fall turns the lower Gauley River into rushing whitewater, creating a popular rafting season. In 2023, it became part of the state park system

Nicholas County has many other points of interest. The Carnifex Ferry Battlefield State Park is an important historic site. The county is known for its annual county fair, the big ramp festival at Richwood, and the potato festival at Summersville. Music in the Mountains, one of America's major bluegrass festivals, is held at Summersville each summer. Nicholas County campgrounds include the Battle Run Campground, Mountain Lake Campground, and the Summersville Lake Retreat. Visitors to the county, which is positioned at the southern end of the Monongahela National Forest, also may enjoy vacation rentals, hiking and biking, boat rentals, horseback riding, scuba diving, whitewater rafting, and visiting the local Kirkwood Winery, home of the Grape Stomping Wine Festival.

In 2016, a deadly flood along the Cherry River devastated Richwood and other locations in the county.