National protected area

• Waccamaw National Wildlife Refuge (part)

State and local protected areas

• Little Pee Dee Heritage Preserve/Wildlife Management Area^[5]
• Marsh Wildlife Management Area^[5]
• Woodbury Wildlife Management Area^[5]

Major water bodies

• Cud Swamp
• Great Pee Dee River
• Lumber River
• Maidendown Swamp
• Waccamaw River

Adjacent counties

• Dillon County – north
• Horry County – east
• Georgetown County – south
• Williamsburg County – southwest
• Florence County – west

Major highways

2020 census

As of the 2020 census, there were 29,183 people, 11,600 households, and 7,532 families residing in the county.

2010 census

At the 2010 census, there were 33,062 people, 13,058 households, and 8,881 families living in the county.^[12][10] The population density was 67.6 inhabitants per square mile (26.1/km²). There were 14,953 housing units at an average density of 30.6 per square mile (11.8/km²).^[13] The racial makeup of the county was 55.9% black or African American, 40.6% white, 0.5% Asian, 0.4% American Indian, 1.3% from other races, and 1.2% from two or more races. Those of Hispanic or Latino origin made up 2.4% of the population.^[12] In terms of ancestry, 7.8% were American, 6.3% were English, and 5.2% were Irish.^[14]

Of the 13,058 households, 33.4% had children under the age of 18 living with them, 38.0% were married couples living together, 24.9% had a female householder with no husband present, 32.0% were non-families, and 28.5% of all households were made up of individuals. The average household size was 2.52 and the average family size was 3.09. The median age was 39.9 years.^[12]

The median income for a household in the county was $30,629 and the median income for a family was $38,043. Males had a median income of $32,414 versus $24,929 for females. The per capita income for the county was $16,653. About 21.4% of families and 25.1% of the population were below the poverty line, including 36.8% of those under age 18 and 19.1% of those age 65 or over.^[15]

2000 census

At the 2000 census,^[16] there were 35,466 people, 13,301 households, and 9,510 families living in the county. The population density was 72 people per square mile (28 people/km²). There were 15,143 housing units at an average density of 31 per square mile (12/km²). The racial makeup of the county was 56.35% Black or African American, 41.69% White, 0.25% Native American, 0.28% Asian, 0.01% Pacific Islander, 0.90% from other races, and 0.52% from two or more races. 1.79% of the population were Hispanic or Latino of any race.

There were 13,301 households, out of which 32.20% had children under the age of 18 living with them, 43.30% were married living together, 23.60% had a female householder with no husband present, and 28.50% were non-families. 25.40% of all households were made up of individuals, and 9.70% had someone living alone who was 65 years of age or older. The average household size was 2.64 and the average family size was 3.16.

In the county, the population was spread out, with 27.60% under the age of 18, 9.70% from 18 to 24, 26.80% from 25 to 44, 23.80% from 45 to 64, and 12.10% who were 65 years of age or older. The median age was 35 years. For every 100 females there were 85.90 males. For every 100 females age 18 and over, there were 80.40 males.

The median income for a household in the county was $26,526, and the median income for a family was $32,932. Males had a median income of $26,133 versus $18,392 for females. The per capita income for the county was $13,878. About 18.90% of families and 23.20% of the population were below the poverty line, including 33.30% of those under age 18 and 23.50% of those age 65 or over.

According to the 2010 U.S. Religious Census, residents of Marion County are predominately Protestant. But the county had the highest concentration of followers of the Baháʼí Faith of any county in the United States, at 5.5%.^[17]

Cities

• Marion (county seat and largest community)
• Mullins

Towns

• Nichols
• Sellers

Census-designated places

• Centenary
• Daviston
• Rains
• Zion

Other unincorporated communities

• Ariel Crossroad
• Brittons Neck
• Friendship
• Gresham
• Temperance Hill