Franklin was the original name of Seward Township. On November 14, 1850, the name changed to Seward, after William H. Seward, who served as governor of New York State and as a U.S. Senator from New York as well as Secretary of State in the Lincoln Administration.
According to the 2010 census, the township has a total area of 35.11 square miles (90.9 km2), of which 34.97 square miles (90.6 km2) (or 99.60%) is land and 0.14 square miles (0.36 km2) (or 0.40%) is water.
It contains portions of Minooka and Joliet. U.S. Route 52 crosses the township east to west, and I-80 runs along the southern boundary of the township.