As of the census of 2000, there were 45,417 people, 16,765 households, and 12,188 families residing in the county. The population density was 71 people per square mile (28/km²). There were 19,191 housing units at an average density of 30 per square mile (12/km²). The racial makeup of the county was 96.06% White, 0.30% Black or African American, 1.51% Native American, 0.26% Asian, 0.02% Pacific Islander, 0.21% from other races, and 1.64% from two or more races. 0.59% of the population were Hispanic or Latino of any race. 18.0% were of French Canadian, 17.8% French, 17.4% American, 12.8% English and 9.8% Irish ancestry according to Census 2000. 94.2% spoke English and 4.8% French as a first language.
There were 16,765 households out of which 37.70% had children under the age of 18 living with them, 58.40% were married couples living together, 9.90% had a female householder with no husband present, and 27.30% were non-families. 20.60% of all households were made up of individuals and 8.40% had someone living alone who was 65 years of age or older. The average household size was 2.67 and the average family size was 3.08.
In the county, the population was spread out with 28.10% under the age of 18, 7.00% from 18 to 24, 31.40% from 25 to 44, 22.50% from 45 to 64, and 11.00% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 98.50 males. For every 100 females age 18 and over, there were 95.90 males.
As of the 2000 RCMS, the three largest denominational groups in Franklin County are Catholic, Mainline Protestant, and Orthodox. The Catholic Church has the highest number of adherents in Franklin County (at 16,280), followed by the United Methodist Church with 2,674 members reported and the Greek Orthodox Archdiocese of Vasiloupulis, reporting 1,720 adherents. The religious body with the largest number of congregations is the United Methodist Church (with 17 congregations) followed by the Catholic Church (with 14 congregations).