Casco Township is a civil township of St. Clair County in the U.S. state of Michigan. As of the 2020 Census, the township population was 3,990. Casco Township was established in 1849. According to the United States Census Bureau, the township has a total area of 37.1 square miles, of which 37.1 square miles is land and 0.04 square miles is water.