Marinette County is part of the Marinette, WI–MI Micropolitan Statistical Area.
As of the census of 2000, there were 43,384 people, 17,585 households, and 11,834 families residing in the county. The population density was 31 people per square mile (12/km²). There were 26,260 housing units at an average density of 19 per square mile (7/km²). The racial makeup of the county was 98.08% White, 0.23% Black or African American, 0.50% Native American, 0.27% Asian, 0.02% Pacific Islander, 0.21% from other races, and 0.69% from two or more races. 0.75% of the population were Hispanic or Latino of any race. 37.8% were of German, 11.7% Polish, 6.4% French and 5.6% American ancestry according to Census 2000.
There were 17,585 households out of which 28.80% had children under the age of 18 living with them, 56.40% were married couples living together, 7.40% had a female householder with no husband present, and 32.70% were non-families. 28.30% of all households were made up of individuals and 13.30% had someone living alone who was 65 years of age or older. The average household size was 2.38 and the average family size was 2.92.
In the county, the population was spread out with 23.50% under the age of 18, 8.10% from 18 to 24, 25.90% from 25 to 44, 25.00% from 45 to 64, and 17.60% who were 65 years of age or older. The median age was 40 years. For every 100 females there were 97.50 males. For every 100 females age 18 and over, there were 95.00 males.