According to the U.S. Census Bureau, the county has a total area of 1,576 square miles (4,080 km2), of which 1,545 square miles (4,000 km2) are land and 31 square miles (80 km2) (1.98%) are covered by water. Marathon County is the largest county in Wisconsin by land area. Founded in 1807, it is also the oldest established County in the State of Wisconsin. Marathon County predates the state of Wisconsin, which was established in 1848, by 41 years.
There were 47,702 households out of which 34.00% had children under the age of 18 living with them, 59.90% were married couples living together, 7.40% had a female householder with no husband present, and 29.00% were non-families. 23.60% of all households were made up of individuals and 9.50% had someone living alone who was 65 years of age or older. The average household size was 2.60 and the average family size was 3.11.
In the county, the population was spread out with 26.80% under the age of 18, 8.20% from 18 to 24, 29.50% from 25 to 44, 22.50% from 45 to 64, and 13.00% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 99.50 males. For every 100 females age 18 and over, there were 97.40 males.