As of the census of 2000, there were 10,906 people, 4,556 households, and 2,870 families residing in the township. The population density was . There were 5,007 housing units at an average density of . The racial makeup of the township was 98.33% White, 0.48% African American, 0.03% Native American, 0.80% Asian, 0.09% from other races, and 0.28% from two or more races. Hispanic or Latino of any race were 0.51% of the population.
There were 4,556 households, out of which 23.5% had children under the age of 18 living with them, 48.4% were married couples living together, 10.5% had a female householder with no husband present, and 37.0% were non-families. 33.1% of all households were made up of individuals, and 16.1% had someone living alone who was 65 years of age or older. The average household size was 2.26 and the average family size was 2.89.
In the township the population was spread out, with 17.6% under the age of 18, 6.4% from 18 to 24, 26.0% from 25 to 44, 25.6% from 45 to 64, and 24.4% who were 65 years of age or older. The median age was 45 years. For every 100 females, there were 93.8 males. For every 100 females age 18 and over, there were 90.1 males.
The median income for a household in the township was $36,603, and the median income for a family was $46,549. Males had a median income of $34,076 versus $25,880 for females. The per capita income for the township was $19,712. About 6.9% of families and 8.3% of the population were below the poverty line, including 11.1% of those under age 18 and 9.5% of those age 65 or over.
Government
The Plains Township Board of Commissioners is the municipality's governing body.
- Thomas Shubilla – Chairman
- Peter J. Biscontini, Esq. – Vice Chairman
- Ciro Cinti
- Robert Sax
- Dennis Condusta
- Attorney Stephen Menn – Solicitor
- Lynell Ent – Township Secretary
- Sharon Stark – Assistant Secretary
Notable people
- Ed Walsh, Chicago White Sox hall of famer
- John J. Yeosock, a United States Army general