According to the 2000 census, the county has a total area of 618.60 square miles (1,602.2 km2), of which 610.86 square miles (1,582.1 km2) (or 98.75%) is land and 7.74 square miles (20.0 km2) (or 1.25%) is water. The Minnesota River flows along the county's northern boundary; two of its tributaries, the Cottonwood and Little Cottonwood Rivers, flow eastwardly through the county.
There were 10,598 households out of which 31.50% had children under the age of 18 living with them, 57.30% were married couples living together, 6.90% had a female householder with no husband present, and 32.40% were non-families. 29.00% of all households were made up of individuals and 14.30% 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 3.00.
In the county the population was spread out with 25.30% under the age of 18, 9.70% from 18 to 24, 25.60% from 25 to 44, 21.90% from 45 to 64, and 17.50% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 98.20 males. For every 100 females age 18 and over, there were 95.90 males.
The median income for a household in the county was $39,800, and the median income for a family was $49,811. Males had a median income of $32,347 versus $23,918 for females. The per capita income for the county was $19,535. About 4.40% of families and 6.40% of the population were below the poverty line, including 7.00% of those under age 18 and 9.80% of those age 65 or over.
In recent years, average temperatures in the county seat of New Ulm have ranged from a low of 6 °F(−14 °C) in January to a high of 83 °F(28 °C) in July, although a record low of −37 °F(−38 °C) was recorded in January 1984 and a record high of 105 °F(41 °C) was recorded in July 1988. Average monthly precipitation ranged from 0.64 inches (16 mm) in January to 4.82 inches (122 mm) in June.