The city of Delafield is a separate municipality from the Town of Delafield, both of which, along with the Village of Nashotah, and the southern part of the Town of Hartland, are situated in the geographical township of Delafield.
According to the United States Census Bureau, the city has a total area of 11.1 square miles (28.7 km²), of which, 9.5 square miles (24.7 km²) of it is land and 1.6 square miles (4.1 km²) of it (14.32%) is water.
There were 2,553 households out of which 35.8% had children under the age of 18 living with them, 61.7% were married couples living together, 8.0% had a female householder with no husband present, and 27.3% were non-families. About 22.2% of all households were made up of individuals and 7.7% had someone living alone who was 65 years of age or older. The average household size was 2.52 and the average family size was 2.97.
In the city the population was spread out with 26.6% under the age of 18, 5.7% from 18 to 24, 29.8% from 25 to 44, 27.1% from 45 to 64, and 10.8% who were 65 years of age or older. The median age was 39 years. For every 100 females there were 98.5 males. For every 100 females age 18 and over, there were 93.3 males.
The median income for a household in the city was $61,938, and the median income for a family was $71,955. Males had a median income of $51,656 versus $30,253 for females. The per capita income for the city was $31,602. About 2.3% of families and 3.4% of the population were below the poverty line, including 5.3% of those under age 18 and 1.4% of those age 65 or over.