The county is named after the strait between the Door Peninsula and Washington Island. The dangerous passage, now littered with shipwrecks, was known to early French explorers and local Native Americans. Because of the natural hazards of the strait, they gave it the French appellation Porte des Morts Passage, which in English means the "Door to the Way to Death," or simply, "Death's Door."
As of the census of 2000, there were 27,961 people, 11,828 households, and 7,995 families residing in the county. The population density was 58 people per square mile (22/km²). There were 19,587 housing units at an average density of 41 per square mile (16/km²). The racial makeup of the county was 97.84% White, 0.19% Black or African American, 0.65% Native American, 0.29% Asian, 0.01% Pacific Islander, 0.33% from other races, and 0.69% from two or more races. 0.95% of the population were Hispanic or Latino of any race. 39.4% were of German and 10.3% Belgian ancestry according to Census 2000. A small pocket of Walloon speakers is the only Walloon-language region outside of Wallonia and its immediate neighborhood.
There were 11,828 households out of which 26.90% had children under the age of 18 living with them, 58.10% were married couples living together, 6.50% had a female householder with no husband present, and 32.40% were non-families. 28.10% of all households were made up of individuals and 12.70% had someone living alone who was 65 years of age or older. The average household size was 2.33 and the average family size was 2.84.
In the county, the population was spread out with 22.10% under the age of 18, 6.10% from 18 to 24, 25.40% from 25 to 44, 27.70% from 45 to 64, and 18.70% who were 65 years of age or older. The median age was 43 years. For every 100 females there were 97.10 males. For every 100 females age 18 and over, there were 94.50 males.