Marion County lies in north-central West Virginia. The area was once part of the original vast Augusta County, Virginia, and was later split between Monongalia and Harrison counties. Interstate 79, U.S. 250, and U.S. 19 intersect in Marion County, at Fairmont, the county seat. The county has an area of 311.6 square miles. The Monongahela River is born in Marion County, with the convergence of the West Fork and Tygart Valley River at Fairmont.

Early settlers began to move into present Marion County after the end of the French and Indian War. Prominent among them were Capt. James Booth, Jacob Prickett, and David Morgan. The Indian threat created the need for frontier forts and several were erected. Pricketts Fort was the most important of these forts and has been reconstructed in recent years as a tourist attraction and state park.

In 1787, Boaz Fleming led a party of relatives and friends from Milford, Delaware, across the mountains to the area of present Fairmont. The group settled there and later found that some of them resided in Monongalia County and the others in Harrison County. Fleming started a petition to have a new county created. Failing in his first attempt, he tried again in the early 1820s after he had founded Middletown (present Fairmont) to be the county seat of the proposed county. His second attempt also failed, and Boaz Fleming died without seeing his desired county established.

Finally in 1842, William S. Morgan, a delegate from Monongalia County, introduced a bill in the Virginia House of Delegates to create a new county from parts of Monongalia and Harrison counties. The county was established on January 14, 1842, and named after Revolutionary War hero Francis Marion. Middletown was chosen as the county seat of Marion County, and its name was changed to Fairmont the following year.

Marion County supplied important leaders in the West Virginia statehood movement. Francis H. Pierpont helped to establish the Reorganized Government of Virginia in Wheeling and served as its governor until 1868. Pierpont practiced law in Fairmont and is buried there. Farmington native Waitman T. Willey served as senator for the Reorganized Government of Virginia and was one of the first two U.S. senators from West Virginia. Four West Virginia governors were Marion County natives: Aretas Brooks Fleming, Ephraim F. Morgan, Matthew M. Neely, and Joe Manchin.

The main line of the Baltimore & Ohio Railroad traversed Marion County in the early 1850s. The arrival of the railroad transformed the region, making it a place of strategic importance during the Civil War and preparing the way for the exploitation of the county's natural resources in the post-war period. Great fortunes were made in coal and oil and gas during the late 19th and early 20th centuries, and the remnants of this wealth may be seen today in the grand houses on Fairmont's Fairmont Avenue. Prominent among the new generation of business leaders were the intermarried Flemings and Watsons, who founded Consolidation Coal Company and produced a West Virginia governor and U.S. senator, the Hutchinsons, and others. James E. Watson's palatial High Gate mansion and other structures associated with Marion County's industrial elite are now listed in the National Register of Historic Places.

Unfortunately, industrialization exacted a high cost. West Virginia's two most notorious coal mine disasters happened a few miles apart in Marion County, at Monongah in 1907 and Farmington in 1968. Farmington claimed 78 lives and Monongah a staggering minimum of 361. Both disasters were followed by significant new federal legislation intended to improve mine safety.

Marion County boomed during the early decades of the 20th century, with the population doubling between 1900 and 1930. The county's industrial economy peaked about the time of World War II. Like many other parts of West Virginia, the county has lost population since 1950, declining from a high that year of 71,521 to 56,205 in 2020. Large investments in the high technology office park outside Fairmont have brought jobs and a modern focus to the local economy. The county also is the home of Fairmont State University.

In 2023, Marion County's largest employers were the county school system, American Consolidated Natural Resources, Fairmont State University, WVU Medicine and Walmart.

Coach Camden Eli "Cam" Henderson and football heroes Robert L. "Sam" Huff and Frank "Gunner" Gatski were Marion Countians. Fairmont native Mary Lou Retton thrilled the nation when she became an Olympic gold medalist in gymnastics in 1984, and today, a city street is named for her. Writer John Knowles, best known for the novel A Separate Peace, was a native Marion Countian. His 1978 novel, A Vein of Riches, is set in a fictionalized Fairmont. National championship-winning football coach Nick Saban was born in Fairmont and attended Monongah High School.