Sinclair Township was named for a government surveyor.

As of the census[1] of 2000, there were 175 people, 79 households, and 51 families residing in the township.

There were 79 households, out of which 24.1% had children under the age of 18 living with them, 58.2% were married couples living together, 3.8% had a female householder with no husband present, and 34.2% were non-families.

30.4% of all households were made up of individuals, and 17.7% had someone living alone who was 65 years of age or older.

About 3.6% of families and 11.8% of the population were below the poverty line, including 27.8% of those under the age of eighteen and 17.6% of those 65 or over.