According to the U.S. Census Bureau, the county has a total area of, of which is land and is water. The Mecan River, Buffalo Lake, and Puckaway Lake lie within Marquette County. The highest altitude in the county is a rocky area known as Mt. Shaw.
As of the census of 2000, there were 15,832 people, 5,986 households, and 4,166 families residing in the county. The population density was 35 people per square mile. There were 8,664 housing units at an average density of 19 per square mile. The racial makeup of the county was 93.66% White, 3.44% Black or African American, 1.04% Native American, 0.27% Asian, 0.10% Pacific Islander, 0.38% from other races, and 1.11% from two or more races. 2.66% of the population were Hispanic or Latino of any race. 45.7% were of German, 8.2% Irish, 6.9% Polish, 6.1% English, 5.6% Norwegian and 5.1% American ancestry. 94.8% spoke English, 2.8% Spanish and 1.1% German as their first language. There were 5,986 households out of which 26.90% had children under the age of 18 living with them, 58.70% were married couplesliving together, 6.70% had a female householder with no husband present, and 30.40% were non-families. 25.40% of all households were made up of individuals and 12.30% had someone living alone who was 65 years of age or older. The average household size was 2.41 and the average family size was 2.86. In the county, the population was spread out with 21.10% under the age of 18, 6.70% from 18 to 24, 28.90% from 25 to 44, 25.00% from 45 to 64, and 18.30% who were 65 years of age or older. The median age was 41 years. For every 100 females there were 118.90 males. For every 100 females age 18 and over, there were 123.70 males. In 2017, there were 133 births, giving a general fertility rate of 65.1 births per 1000 women aged 15–44, the 29th highest rate out of all 72 Wisconsin counties. Of these, 11 of the births occurred at home. Additionally, there were 17 reported induced abortions performed on women of Marquette County residence in 2017, a figure higher than the records for the preceding four years.
