Fairmont is located in north-central West Virginia, where the West Fork and Tygart Valley rivers join to form the Monongahela. Fairmont was established by the Virginia legislature on January 19, 1820, on the farm of Boaz Fleming. It became the county seat when Marion County was created in 1842. The original name of Middletown was changed to Fairmont in 1843.

Neighboring Palatine was surveyed in 1838 and incorporated in 1867. A number of industries developed in Palatine, and it once outranked Fairmont in size and importance. Late in the 19th century, the Fairmont Development Company began developing the area southwest of Fairmont. The area was leveled and utilities and paved streets installed. Businesses and residences were constructed quickly, and on December 15, 1892, West Fairmont was incorporated. The present city was born February 18, 1899, when a new charter was granted by which Fairmont, Palatine, and West Fairmont were incorporated as the city of Fairmont.

Early in the 20th century, Fairmont developed a major glass industry, based on the availability of glass sand and abundant natural gas. Experienced glass workers were brought in to operate the plants and train new workers. Many came directly from Belgium, France, and Italy, while others, who had earlier immigrated to work at Pennsylvania or Ohio glass plants, were persuaded to transfer to Fairmont.

Local coal mines and other industries also developed a pressing need for labor. African-Americans from the American South and immigrants from southern and eastern Europe came to the region. The immigrants included Italians, Poles, Hungarians, Germans, and Austrians, and some from Turkey, Greece, and elsewhere. Experienced miners from England and Wales were brought over to teach proper methods to the new miners. Each group brought its skills and knowledge, or simply its muscle power, to the growing local economy. All contributed to the enrichment of what is today a diverse ethnic culture.

Businesses flourished in Fairmont, with Monongah Glass, Owens-Illinois Glass, and Westinghouse Electric prominent among them. The town was headquarters for Fairmont Coal Company and later for Mountaineer Coal Division of Consolidation Coal Company. Today, it is the home of Fairmont State University and two high schools. Cook Hospital was formed here, and eventually became Fairmont General.

As in many cities, Fairmont suffered with the loss of industry and as retail business moved to outlying shopping centers. The population peaked at 29,346 in 1950 and was an estimated 18,737 in 2012. Efforts have been made to revive the downtown, including the restoration of the historic high-level bridge and the beautiful Marion County courthouse. Major investments have been made on the city's outskirts, particularly the Fairmont Technology Park. Fairmont benefits as well from the huge FBI fingerprint center on I-79 south, and from other developments on the emerging Morgantown-Clarksburg technology corridor.

Fairmont is the home of Alan Mollohan, who served in the U.S. Congress from 1982 to 2011. Francis Harrison Pierpont, governor of the Reorganized Government of Virginia during the Civil War, came from Fairmont, as did four governors of West Virginia, Aretas Brooks Fleming, Ephraim F. Morgan, Matthew M. Neely, and Joe Manchin. Fairmont was the home of James O. Watson and his son, U.S. Sen. Clarence W. Watson, and son-in-law, Gov. Aretas Brooks Fleming; for many years these men controlled Consolidation Coal Company.

These industrialists and others built great mansions on Fairmont Avenue. Most of the grand houses have succumbed to time but interesting examples remain, including James Edwin Watson's palatial High Gate. Novelist John Knowles, a Fairmont native best known for the novel, A Separate Peace, dealt with this period of local history in the 1978 book, A Vein of Riches. Knowles used the original name of Middletown in fictionalizing Fairmont.