Cain Township was one of the first townships to be established in the county; it was established on July 24, 1826. At that time, much of the land was forested, but 100 years later most of the land was used for agriculture. There were also several grain and lumber mills along Coal Creek, but these also were gone by the early 20th century. The town of Hillsboro was laid out in the 1830s and still exists; the town of Rynear was established in the far northwest corner of the township, but was gone by the early 1900s. The township's population was 1,763 in 1880, 1,537 in 1900, and 1,403 in 1910.
According to the 2010 census, the township has a total area of 39.96 square miles (103.5 km2), of which 39.93 square miles (103.4 km2) (or 99.92%) is land and 0.03 square miles (0.078 km2) (or 0.08%) is water. There is a single settlement in the township: the town of Hillsboro, which has a population of about 500 (about half the township's total population). This lies at the intersection of the east–west U.S. Route 136 and the north–south Indiana State Road 341, in the northern part of the township. Interstate 74 passes from east to west less than 1 mile (1.6 km) north of Hillsboro.Indiana State Road 32 runs along a portion of the southern border of the township.