Marion Township is located in Ogle County, Illinois. As of the 2010 census, its population was 4,135 and it contained 1,531 housing units. This population dropped in 2022 when a population of 3,898 people was recorded.
According to the 2010 census, the township has a total area of , of which (or 98.72%) is land and (or 1.25%) is water.