According to the U.S. Census Bureau, the county has a total area of, of which are land and are covered by water. It is the third-smallest county in Wisconsin by total area.
As of the census of 2000, there were 19,105 people, 7,703 households, and 5,322 families residing in the county. The population density was 54 people per square mile. There were 9,831 housing units at an average density of 28 per square mile. The racial makeup of the county was 97.81% White, 0.15% Black or African American, 0.20% Native American, 0.31% Asian, 0.04% Pacific Islander, 0.89% from other races, and 0.60% from two or more races. 2.06% of the population were Hispanic or Latino of any race. 51.8% were of German, 10.6% Polish, 5.8% Irish and 5.8% American ancestry. 94.2% spoke English, 3.0% Spanish and 1.8% German as their first language. There were 7,703 households out of which 29.40% had children under the age of 18 living with them, 58.50% were married couplesliving together, 6.90% had a female householder with no husband present, and 30.90% were non-families. 27.00% of all households were made up of individuals and 13.80% had someone living alone who was 65 years of age or older. The average household size was 2.43 and the average family size was 2.96. In the county, the population was spread out with 24.20% under the age of 18, 6.60% from 18 to 24, 26.20% from 25 to 44, 24.20% from 45 to 64, and 18.80% who were 65 years of age or older. The median age was 41 years. For every 100 females there were 97.00 males. For every 100 females age 18 and over, there were 95.20 males. In 2017, there were 380 births, giving a general fertility rate of 61.6 births per 1000 women aged 15–44, the 32nd lowest rate out of all 72 Wisconsin counties. Of these, 77 of the births occurred at home, the fourth highest for Wisconsin counties. Additionally, there were 11 reported induced abortions performed on women of Green Lake County residence in 2017. In 2010, the largest religious groups by number of adherents were Catholic at 5,290 adherents, Wisconsin Synod Lutheran at 2,498 adherents, Missouri Synod Lutheran at 1,173 adherents, ELCA Lutheran at 964 adherents, and Amish at 812 adherents.
