St. Mary Township was organized in 1858, taking the name of a local Roman Catholic church.

As of the census[1] of 2000, there were 504 people, 162 households, and 134 families residing in the township.

The population density was 14.1 people per square mile (5.4/km2).

There were 162 households, out of which 45.1% had children under the age of 18 living with them, 79.6% were married couples living together, 2.5% had a female householder with no husband present, and 16.7% were non-families.

13.6% of all households were made up of individuals, and 3.7% had someone living alone who was 65 years of age or older.