According to the United States Census Bureau, the city has a total area of 3.4 square miles (8.8km2); of this, 0.01 square miles (0.3%) is water. Anna is a city in Union County, Illinois, United States. Located in Southern Illinois. The city is known for being tied to its close neighbor Jonesboro, together known as Anna-Jonesboro. Anna is well renowned for the Anna State Mental Hospital or the Choate Mental Health Care Center. Anna is also a historical sundown town.