Adjacent counties

• Etowah County - northeast
• Calhoun County - east
• Talladega County - southeast
• Shelby County - southwest
• Jefferson County - west
• Blount County - northwest

2020 census

As of the census of 2020, there were 91,103 people, 32,829 households, and 25,409 families residing in the county.

2010 census

As of the census of 2010, there were 83,593 people, 31,624 households, and 23,364 families living in the county. The population density was 132 people per square mile (51 people/km²). There were 35,541 housing units at an average density of 56 units per square mile (22 units/km²). The racial makeup of the county was 88.2% White, 8.6% Black or African American, 0.3% Native American, 0.6% Asian, 0.1% Pacific Islander, 0.9% from other races, and 1.3% from two or more races. 2.1% of the population were Hispanic or Latino of any race.^[16] Of the 31,624 households 30.7% had children under the age of 18 living with them, 58.3% were married couples living together, 11.2% had a female householder with no husband present, and 26.1% were non-families. 22.5% of households were one person and 8.5% were one person aged 65 or older. The average household size was 2.58 and the average family size was 3.02.

The age distribution was 23.7% under the age of 18, 7.8% from 18 to 24, 27.8% from 25 to 44, 27.6% from 45 to 64, and 13.1% 65 or older. The median age was 38.6 years. For every 100 females, there were 100.5 males. For every 100 females age 18 and over, there were 102.3 males.

The median household income was $48,837 and the median family income was $56,107. Males had a median income of $43,287 versus $32,843 for females. The per capita income for the county was $22,192. About 8.3% of families and 10.6% of the population were below the poverty line, including 13.2% of those under age 18 and 9.5% of those age 65 or over.

2000 census

As of the census of 2000, there were 64,742 people, 24,143 households, and 18,445 families living in the county. The population density was 102 people per square mile (39 people/km²). There were 27,303 housing units at an average density of 43 units per square mile (17 units/km²). The racial makeup of the county was 90.03% White, 8.13% Black or African American, 0.37% Native American, 0.17% Asian, 0.03% Pacific Islander, 0.41% from other races, and 0.85% from two or more races. 1.06% of the population were Hispanic or Latino of any race.^[17] In 2000 the largest ancestry groups in St. Clair county were:

• English 71%
• Irish 13.1%
• African 8.13%
• German 8%
• Scots-Irish 3.5%
• Dutch 2.4%
• Scottish 2%

Of the 24,143 households 35.10% had children under the age of 18 living with them, 62.80% were married couples living together, 10.00% had a female householder with no husband present, and 23.60% were non-families. 20.80% of households were one person and 8.20% were one person aged 65 or older. The average household size was 2.60 and the average family size was 3.01.

The age distribution was 25.40% under the age of 18, 7.90% from 18 to 24, 30.70% from 25 to 44, 24.30% from 45 to 64, and 11.70% 65 or older. The median age was 36 years. For every 100 females, there were 101.80 males. For every 100 females age 18 and over, there were 98.80 males.

The median household income was $37,285 and the median family income was $43,152. Males had a median income of $33,914 versus $24,433 for females. The per capita income for the county was $17,960. About 9.60% of families and 12.10% of the population were below the poverty line, including 15.20% of those under age 18 and 12.60% of those age 65 or over.

Cities

• Ashville (county seat)
• Leeds (mostly in Jefferson County and Shelby County)
• Margaret
• Moody
• Pell City (county seat)
• Springville
• Trussville (mostly in Jefferson County)

Towns

Unincorporated communities

Former towns

Major highways

Rail

• Alabama and Tennessee River Railway
• Norfolk Southern Railway

Historically, the Southern Railway ran several daily passenger trains, including the Kansas City-Florida Special and an Atlanta-Birmingham section of the Piedmont Limited, making stops in Pell City. The Sunnyland made signal stops as well.^[19] The last trains made stops in 1967.^[20][21] Today, the nearest passenger service is Amtrak's Crescent in Anniston 30.6 miles to the east.