Iosco is a Native American word meaning "water of light" In 1840 it was set off as Kanotin County, and renamed Iosco in 1843 The county was organized in 1857.
As of the census of 2000, there were 27,339 people, 11,727 households, and 7,857 families residing in the county. The population density was 50 people per square mile (19/km²). There were 20,432 housing units at an average density of 37 per square mile (14/km²). The racial makeup of the county was 96.92% White, 0.41% Black or African American, 0.66% Native American, 0.46% Asian, 0.05% Pacific Islander, 0.23% from other races, and 1.27% from two or more races. 0.98% of the population were Hispanic or Latino of any race. 23.2% were of German, 12.3% English, 10.6% Irish, 9.9% American, 8.3% Polish and 7.1% French ancestry according to Census 2000. 97.4% spoke English and 1.0% Spanish as their first language.
There were 11,727 households out of which 24.90% had children under the age of 18 living with them, 55.20% were married couples living together, 8.40% had a female householder with no husband present, and 33.00% were non-families. 28.60% of all households were made up of individuals and 14.00% had someone living alone who was 65 years of age or older. The average household size was 2.30 and the average family size was 2.79.
In the county the population was spread out with 22.40% under the age of 18, 5.40% from 18 to 24, 23.40% from 25 to 44, 27.30% from 45 to 64, and 21.60% who were 65 years of age or older. The median age was 44 years. For every 100 females there were 96.30 males. For every 100 females age 18 and over, there were 93.60 males.
The median income for a household in the county was $31,321, and the median income for a family was $37,452. Males had a median income of $30,338 versus $21,149 for females. The per capita income for the county was $17,115. About 9.50% of families and 12.70% of the population were below the poverty line, including 18.50% of those under age 18 and 7.60% of those age 65 or over.