Sullivan Township is located in Livingston County, Illinois. As of the 2010 census, its population was 724 and it contained 354 housing units. Sullivan Township formed from Saunemin Township in 1863.
According to the 2010 census, the township has a total area of , all land.
The township contains the village of Cullom and the former communities of Griswold, Sullivan Center and Saxony.