Cassella is situated in Southern Mercer County, in Marion Township. State Route 119 runs through the community. Cassella is in the Marion Local School District.

Cassella was first named Dogtown. In 1860 it was named Marysville after the chapel, Maria-Kapelle. When Marysville was first laid out, the Post Office was called Kapelle. A government official misunderstood Kapelle, and the Post Office was given the name of Cassella. Later, the community was called Cassella after the Post Office.

Early settlers were from Germany and France. Many settlers earned a living by hunting, trapping and cutting timber. Cassella's early businesses included a saloon, blacksmith shop wagon repair shop, little log schoolhouse, wooden mission church, teacher's house, shoe-repair shop, barber shop and cane mill.