Saint Mary’s Township was most likely given the name by Catholic members of the community who worshiped at St. Mary’s-of-the-Barrens Church.  The name St. Mary's may be further connected to Catholic Kentucky settlers who originated in St. Mary's County, Maryland. 
St. Mary’s Township lies in the southwestern part of Perry County, and was organized between 1850 and 1860. 
St. Mary’s is situated in the southwestern corner of Perry Couty. There are three unincorporated communities in St. Mary’s Township: Barks, Silver Lake and Yount. 
As of the census of 2000, there were 1,681 people residing in the township. The racial makeup of the town was 98.6% White, 0.4% American Indian and Alaska Native, and 0.5% from other races.
As of the census of 2010, there were 1,708 people, with a density of 16.7 per square mile (6.5 km2), residing in the township. The population density was 25 people per square mile. The racial makeup of the town was 98.24% White, 0.53% American Indian and Alaska Native, and 0.88% from other races.