Jenner Township includes the unincorporated communities of Jenners, Jenner Crossroads, Ralphton, Pilltown, Ferrellton, Acosta, and Gray. Jenner Township completely surrounds the nearby boroughs of Boswell and Jennerstown, each of which has its own government and is not part of the Township.
It is located approximately at 40.175°N by 79.06°W. According to the United States Census Bureau, the township has a total area of 65.1 square miles (169 km2), of which 64.6 square miles (167 km2) is land and 0.5 square miles (1.3 km2) (0.77%) is water. Jenner Township is bordered by Conemaugh Township to the northeast, Quemahoning Township to the southeast, Lincoln Township to the southwest, and Westmoreland County to the northwest.
As of the census of 2000, there were 4,054 people, 1,598 households, and 1,192 families residing in the township. The population density was 62.8 people per square mile (24.2/km2). There were 1,773 housing units at an average density of 27.4/sq mi (10.6/km2). The racial makeup of the township was 99.33% White, 0.05% Native American, 0.10% Asian, 0.12% from other races, and 0.39% from two or more races. Hispanic or Latino of any race were 0.25% of the population.
There were 1,598 households, out of which 28.8% had children under the age of 18 living with them, 59.6% were married couples living together, 9.9% had a female householder with no husband present, and 25.4% were non-families. 22.3% of all households were made up of individuals, and 11.4% had someone living alone who was 65 years of age or older. The average household size was 2.54 and the average family size was 2.96.
In the township the population was spread out, with 22.1% under the age of 18, 8.0% from 18 to 24, 27.0% from 25 to 44, 26.7% from 45 to 64, and 16.2% who were 65 years of age or older. The median age was 41 years. For every 100 females there were 98.0 males. For every 100 females age 18 and over, there were 97.6 males.
The median income for a household in the township was $32,141, and the median income for a family was $39,341. Males had a median income of $26,163 versus $22,688 for females. The per capita income for the township was $15,066. About 8.8% of families and 10.2% of the population were below the poverty line, including 14.6% of those under age 18 and 11.7% of those age 65 or over.