Marion County is a county located in the U.S. state of Kansas. Its county seat is Marion and its most populous city is Hillsboro. As of the 2020 census, the county population was 11,823. The county was named in honor of Francis Marion, a brigadier general of the American Revolutionary War, known as the "Swamp Fox".

For millennia, the Great Plains of North America were inhabited by Native Americans. From the 16th to 18th centuries, the Kingdom of France claimed ownership of large parts of North America. In 1762, after the French and Indian War, France secretly ceded New France to Spain, by the Treaty of Fontainebleau.

In 1802, Spain returned most of the land to France, but keeping title to about 7,500 square miles. In 1803, most of the land for modern day Kansas was acquired by the United States from France as part of the 828,000 square mile Louisiana Purchase for 2.83 cents per acre. In 1848, after the Mexican–American War, the Treaty of Guadalupe Hidalgo with Spain brought into the United States all or part of land for ten future states, including southwest Kansas.

In 1806, Zebulon Pike led the Pike Expedition westward from St Louis, Missouri, of which part of their journey followed the Cottonwood River through modern Marion County near the current cities of Florence, Marion, Durham.

In 1854, the Kansas Territory was organized, then in 1861 Kansas became the 34th U.S. state. In 1855, Marion County was established. The first settlers in Marion County located on Doyle Creek, near the present site of Florence. They were Moses Shane, who came in 1858, and whose death the next year was the first in the county; Patrick Doyle, in 1859, for whom Doyle Creek and Township were named, and a family by the name of Welsh, in which occurred the first birth in the county in August 1859. The city of Marion Centre was founded in 1860, became the county seat in 1865, and later the city name was shortened to Marion.

From 1821 to 1866, the Santa Fe Trail was active across Marion County. In the spring of 1859, a trading post was established at the "Lost Spring" on the Santa Fe Trail, and in the autumn of the same year, the Moore brothers established a ranch near the present site of Durham, and the first post office was established at this place. Later in the same year, a post office was established at "Lost Spring" near the current city of Lost Springs. Previously the nearest post office was Emporia.

From 1867 to 1871, the Chisholm Trail was routed along the western edge of Marion County. The trail started in Texas and ended in Abilene, Kansas, where cattle were shipped eastward by rail. As the railroads were built westward and southward, the trail was truncated from going further north.

Originally, Marion County covered more than a third of the area of Kansas, including all the territory in the state south and west of the present northern and eastern lines of the county. The original location of the county was fixed by legislative act in 1860. It comprised less than the present area. The original boundaries were altered by an increase of territory on the west and a decrease on the south. In 1863, the legislature by special act fixed the boundaries to include all of southwestern Kansas. In June of that year, on petition of the citizens of the county, the governor restored the previous boundaries and ordered a separate organization of the county. The south-eastern border one mile "notch" with Chase County was established under unusual circumstances. A murder had occurred and Marion County didn't want to have the trial, so a section one mile wide and eighteen miles long was ceded to Chase County to ensure the murder had occurred there. The one mile strip of land remains in Chase County to this day. The present county boundary lines were decided upon in 1872 and contain twenty-four townships.