The township is in central Livingston County and is bordered to the northeast by the city of Howell, the county seat. Interstate 96 runs along the northeast edge of the township, with access from Exit 137 (Pinckney Road). I-96 leads southeast 54 miles (87 km) to Detroit and northwest 36 miles (58 km) to Lansing.
According to the United States Census Bureau, Marion Township has a total area of 35.8 square miles (92.8 km2), of which 34.9 square miles (90.4 km2) are land and 0.93 square miles (2.4 km2), or 2.61%, are water.[5] The township is part of the watersheds of three different Great Lakes: the eastern side of the township is within the watershed of the South Branch of the Shiawassee River, leading north towards Saginaw Bay on Lake Huron; the western side contains the headwaters of the Red Cedar River, flowing north and west to the Grand River and eventually Lake Michigan; and the southern part of the township is part of the Huron River watershed, leading southeast to Lake Erie.