[3] Mason Township is located in southern Cass County, along the Indiana state line.

The unincorporated community of Kessington is in the southeastern part of the township, and Allenton is in the southwest.

The community of Simonton Lake, Indiana, is directly to the south.

[5] As of the census[1] of 2000, there were 2,514 people, 953 households, and 732 families residing in the township.

16.7% of all households were made up of individuals, and 5.9% had someone living alone who was 65 years of age or older.