Millbrook Township is located in Peoria County, Illinois. As of the 2020 census, its population was 419 and it contained 192 housing units.
As of the 2010 census, its population was 488 and it contained 197 housing units.
According to the 2010 census, the township has a total area of , of which (or 99.73%) is land and (or 0.30%) is water.