Mount Vernon, the county seat of Franklin County, is seventy-two miles southwest of Texarkana and ninety-six miles northeast of Dallas in the central portion of the county. The area was first settled in the mid-1840s, and Joshua Foster Johnson, who arrived in 1845, is generally recognized as the first settler. The town sprang up around a post office established in 1848 with William Stephen Keith as postmaster. It was originally called Keith and was renamed Lone Star in 1850. Although the name was not officially changed to Mount Vernon until September 1875, the town was generally known as Mount Vernon much earlier than that. According to the New Orleans Daily Crescent the town of Mount Vernon was destroyed by fire during the Texas Troubles in the summer of 1860, and one of the individuals suspected of involvement in the fire was caught and hanged (see SLAVE INSURRECTIONS). By 1870 the town had several stores, at least two mills, a school, three churches, and a population of 223. When Franklin County was organized in the spring of 1875 the commissioners appointed by the legislature were directed to meet at Mount Vernon to organize an election to choose county officials and a permanent county seat. Citizens of Mount Vernon pledged to donate more than $3,000 in land and money if their town was chosen county seat. The election was held on April 30, 1875, and Mount Vernon received a large majority of the votes cast, easily defeating Mile Post 14 and Mile Post 15, its two competitors. The county seat was never again contested. By 1876 the town had a newspaper, the Franklin Herald, published by W. J. Johnson. In 1878 he sold the paper to B. W. Billups, who published it until 1880, when it was purchased by James H. "Cyclone" Davis, who later sold it to R. W. Holbrook and C. C. Dupree. In 1906 the Herald was merged with the Mount Vernon Optic, founded in 1894, to become the Mount Vernon Optic-Herald, which was still being published in 1984.
Although the East Line and Red River Railroad had been constructed across the southeastern corner of the county in 1876, Mount Vernon did not obtain a railroad until 1887, when the St. Louis, Arkansas and Texas Railway was built across the center of the county. During the years between the organization of the county and the coming of the railroad, Mount Vernon grew slowly; the population increased from 223 in 1870 to an estimated 350 in 1884. With rail connections the town grew more rapidly, and by 1890 the population had reached an estimated 700. Mount Vernon had become the major supply and shipping center for the county's farmers, as well as the site of a planing mill that served the county's lumbering interests. The town also had a small bank in 1890, owned and operated by J. H. Majors, for whom the community of Majors was named.
Mount Vernon was incorporated in 1910, and its population was estimated at 1,200 by 1914, when the community had an electricity plant, a waterworks, an ice plant, a telephone system, and three banks. The population remained relatively stable from that time through 1960, when it was reported as 1,338. The town remained a supply center for most of the county's dwindling rural population, but in the 1960s efforts were made to bring industry to the town. An industrial park was established by the Franklin County Industrial Foundation, and by 1970 Mount Vernon reported five industries employing from 200 to 350 workers. The population had grown from 1,338 to 1,806. In 1980 the population was reported as 2,025, and the town had fifty-four rated businesses. In 1990 the population was 2,219. The population grew to 2,286 by 2000.