The Township has a Supervisor-Board style township government with elected supervisor, clerk, treasurer and four trustees. School district serving the Township residents are Romeo Community Schools and Utica Community Schools and Rochester Schools from Oakland County.
According to the United States Census Bureau, the township has a total area of 36.8 square miles (95.3 km²), of which 36.0 square miles (93.1 km²) is land and 0.9 square mile (2.2 km²) (2.34%) is water.
Weather in Washington Township is similar to Detroit, generally being only a few degrees cooler allowing for more snow in the winter.
A notable weather event occurred on July 2, 1997 when, during the Michigan Tornado Outbreak, a EF0 tornado touched down at 26 mile Road and destroyed parts of the Rudgate trailer park community. Six people were injured during the events.
As of the census of 2000, there were 19,080 people, 6,991 households, and 5,383 families residing in the township. The population density was 530.7 per square mile (204.9/km²). There were 7,317 housing units at an average density of 203.5 per square mile (78.6/km²). The racial makeup of the township was 97.23% White, 0.49% African American, 0.26% Native American, 0.62% Asian, 0.02% Pacific Islander, 0.58% from other races, and 0.79% from two or more races. Hispanic or Latino of any race were 2.40% of the population.
There were 6,991 households out of which 36.7% had children under the age of 18 living with them, 66.5% were married couples living together, 7.7% had a female householder with no husband present, and 23.0% were non-families. 19.4% of all households were made up of individuals and 5.8% had someone living alone who was 65 years of age or older. The average household size was 2.71 and the average family size was 3.14.
In the township the population was spread out with 26.6% under the age of 18, 7.4% from 18 to 24, 30.4% from 25 to 44, 26.6% from 45 to 64, and 8.9% who were 65 years of age or older. The median age was 37 years. For every 100 females there were 97.8 males. For every 100 females age 18 and over, there were 95.7 males.
The median income for a household in the township was $68,841, and the median income for a family was $78,988. Males had a median income of $60,721 versus $31,213 for females. The per capita income for the township was $30,740. About 2.8% of families and 3.7% of the population were below the poverty line, including 5.0% of those under age 18 and 2.7% of those age 65 or over.