[2] The county was created by an act of the Alabama Territorial General Assembly on February 13, 1818.

The county seat was originally established in Pikeville in 1820, and moved to Hamilton in 1881.

The county was named by planter and US Indian agent John Dabney Terrell, Sr., in recognition of General Francis Marion of South Carolina.

The county was named in honor of General Francis Marion (1732–1795), an American Revolutionary War hero from South Carolina who was known as "The Swamp Fox."

Many early settlers of Marion County came from Kentucky and Tennessee after General Andrew Jackson established the Military Road.

The first towns in the area were Pikeville, Hamilton (formerly named Toll Gate), Winfield, and Guin.

The county's first seat was settled in 1818 at Cotton Gin Port, near present-day Amory, Mississippi.

It was moved in 1819 to the home of Henry Greer along the Buttahatchee River, in 1820, the first permanent county seat was established at Pikeville, now a ghost town, located between present day Hamilton and Guin, along U.S. Highway 43.

Although the town is now abandoned, the home of Judge John Dabney Terrell Sr., which served as the third county courthouse, still stands.

In 1959, the building was significantly remodeled to give the structure its current 1950s "international style" design theme.

[5] As of the 2020 United States census, there were 29,341 people, 11,997 households, and 8,030 families residing in the county.

Hamilton is home to a campus of Bevill State Community College.