[1] Located in the western part of the county, it borders the following townships: Most of the city of St. Marys is located in northern Saint Marys Township, and the eastern end of Grand Lake St. Marys and its respective state park are located in the western part of the township.

[2] According to the U.S. Census Bureau, the total area of the township is 36.9 square miles (95.6 km2).

[4] Originally part of Mercer County, the township was formed in 1824.

[5] The township is governed by a three-member board of trustees, who are elected in November of odd-numbered years to a four-year term beginning on the following January 1.

The northern portion of the township is served by the Saint Marys (45885) post office, with the southern portions being split between the New Bremen (45869) and the New Knoxville (45871) post offices.