Marion Township is located in Lee County, Illinois. As of the 2010 census, its population was 232 and it contained 114 housing units.
Marion Township was formed from Hamilton and Amboy townships in September 1854. Three years later, on March 3, 1857, the western half of the township became Harmon Township allowing Marion Township to take its present form.
According to the 2010 census, the township has a total area of , of which (or 99.97%) is land and (or 0.06%) is water.