According to the U.S. Census Bureau, the township has a total area of 35.79 square miles (92.70 km2), all land.

The racial makeup of the township was 98.77% White, 0.88% Native American, and 0.35% from two or more races.

20.0% of all households were made up of individuals, and 10.7% had someone living alone who was 65 years of age or older.

Slagle Township is served by two public school districts.

The majority of the township is served by Mesick Consolidated Schools to the north in Mesick, while the eastern portion of the township is served by Cadillac Area Public Schools to the east in Cadillac.