Transit

• Marshalltown Municipal Transit

Major highways

• US 30
• Iowa 14
• Iowa 96
• Iowa 146
• Iowa 330

Adjacent counties

Jasper County (South)
Tama County (East)
Story County (West)
Hardin County (Northwest)
Grundy County (North)

2020 census

The 2020 census recorded a population of 40,105 in the county, with a population density of 70.1835/sq mi (27.0980/km²). 90.76% of the population reported being of one race. There were 16,745 housing units, of which 15,358 were occupied.^[1]

2010 census

The 2010 census recorded a population of 40,648 in the county, with a population density of 71.0232/sq mi (27.4222/km²). There were 16,831 housing units, of which 15,538 were occupied.^[13]

2000 census

At the 2000 census there were 39,311 people, 15,338 households, and 10,460 families in the county. The population density was 69 people per square mile (27 people/km²). There were 16,324 housing units at an average density of 28 units per square mile (11 units/km²). The racial makeup of the county was 90.44% White, 0.93% Black or African American, 0.35% Native American, 0.78% Asian, 0.06% Pacific Islander, 6.03% from other races, and 1.42% from two or more races. 8.96%.^[14] were Hispanic or Latino of any race.

Of the 15,338 households 31.10% had children under the age of 18 living with them, 55.40% were married couples living together, 9.30% had a female householder with no husband present, and 31.80% were non-families. 26.90% of households were one person and 12.20% were one person aged 65 or older. The average household size was 2.48 and the average family size was 3.00.

The age distribution was 25.30% under the age of 18, 8.10% from 18 to 24, 26.30% from 25 to 44, 23.90% from 45 to 64, and 16.40% 65 or older. The median age was 39 years. For every 100 females there were 99.20 males. For every 100 females age 18 and over, there were 96.50 males.

The median household income was $38,268 and the median family income was $46,627. Males had a median income of $33,809 versus $24,063 for females. The per capita income for the county was $19,176. About 7.10% of families and 10.20% of the population were below the poverty line, including 14.30% of those under age 18 and 8.60% of those age 65 or over.

Cities

Census-designated place

• Green Mountain

Other unincorporated communities

• Bangor
• Dunbar
• LaMoille
• Marietta
• Quarry

Townships

Population ranking

The population ranking of the following table is based on the 2020 census of Marshall County.^[1]

† county seat