Pleasants County, named for James Pleasants, governor of Virginia (1822—24), is one of the smallest counties in West Virginia. It has 134.6 square miles and an estimated 2012 population of 7,595. Pleasants County was created in 1851 from portions of Wood, Ritchie, and Tyler counties.

Pleasants County is located on the Ohio River northeast of Parkersburg. The main roadways are State Route 2 and State Route 16. Middle Island Creek, French Creek, Cow Creek, and Bull Creek are four major streams entering the Ohio River in Pleasants County. Five Ohio River islands are within the county boundaries. The mostly rural county is dotted with small communities such as Schultz, Horseneck, Calcutta, Arvilla, Hebron, Nine Mile, Mt. Carmel, Shawnee, and Henry Camp. St. Marys and Belmont are the only incorporated places in Pleasants County, with populations of 1,860 (2010) and 903 (2010) respectively. St. Marys is the county seat.

The earliest permanent settlers were Jacob and Isaac LaRue, in 1797. Soon Riggses, Reynoldses, Tripletts, Smiths, Baileys, and others settled in the area. The person who was most prominent in the creation of the county was Alexander H. Creel. In 1843, he established a river port village named Vaucluse at the mouth of Green Run just south of present St. Marys. He constructed a road linking Vaucluse with the Northwestern Turnpike 13 miles away at Pike. At its height Vaucluse rivaled Parkersburg as a major port on the Ohio, and many businesses built warehouses there. However, a massive fire and the flood of 1848 doomed Creel's village to an early death.

In 1849, Creel bought back land he had sold to Hugh Pickens and established the city of St. Marys. At the time there was talk of bringing a railroad down Middle Island Creek to the town site. When the county was created, the first county court was held in Creel's house. He donated the property on which the first Pleasants County courthouse was built, now the site of the second courthouse.

In early years, agriculture was the main source of income. The oil boom of the 1860s brought new opportunities. The first gusher was struck at Horseneck, near the Wood County border in the Hendershot oil field. Because oil had to be transported in wooden barrels, cooper shops, once numbering as many as 13, sprang up in St. Marys. Oil and gas have remained important to the Pleasants County economy to the present. The industry has boomed several times, including the 1890s and 1980s.

Important oil industry technologies were developed or improved in Pleasants County. In 1864, Bazel Childers developed a system for fracturing the oil-bearing rock to increase production by the use of explosives deep in the well. In the early 20th century, James Dinsmoor came into the area determined to prove that more oil could be retrieved from wells previously considered exhausted. His secondary recovery system was a big success and one of the reasons for the establishment of the Ohio Valley Refining Company at St. Marys in 1913. The refinery was an important employer for Pleasants County until it was closed in 1987.

Pleasants County history has been greatly influenced by its location on the Ohio River and its proximity to the railroad. In 1882, the Baltimore & Ohio Railroad decided to extend down the Ohio from Wheeling to Parkersburg. When completed, the rail line opened transportation and communication for the county regardless of weather or river conditions.

After World War II, large industries moved into the county. One of the most significant was American Cyanamid at Willow Island in 1946. Others soon followed, including the Cabot Corporation in the southern part of the county. Construction climaxed in the late 1970s with the building of the Pleasants Power (now Allegheny Energy) plant at Willow Island. This construction project brought the most tragic day in the history of Pleasants County. On April 27, 1978, 51 men fell to their deaths in the collapse of a cooling tower that was being built.

The county's economy has changed in recent years. Many retail businesses have been lost to competition from larger stores in the Parkersburg and Marietta area. The Colin Anderson Center, a state facility for the mentally retarded located three miles north of St. Marys, was closed and reopened as a medium-security prison. The Dominion Hope gas company opened a gas-fired electricity generating plant at Bull Creek in 2002. This brings the number of power plants to three, counting the two of the Willow Island site, making Pleasants County one of West Virginia's leading electricity producers.

Notable natives of Pleasants County include Hiram A. Carpenter, river man, farmer, and contractor; Brooks F. Ellis, educator and petroleum geologist; West Virginia historian Charles H. Ambler; Congressman Cleveland M. Bailey; and John Deaver Drinko, attorney and philanthropist.