Hannelius was born in Boston, Massachusetts. She moved to southern Maine at age three. She started acting at a young age. Her theater roles include playing Madeline in the Children's Theater of Maine's production of Madeline's Rescue in summer 2002. and Jenny in Tales of a Fourth Grade Nothing with Children's Theatre of Maine and Maine State Music Theater. She moved to California for three months to try out auditions, and she was so successful that her family relocated permanently to Los Angeles in 2008. Hannelius is an alumna of the Los Angeles-based Young Actor's Studio.
In February 2011, she was cast for a lead role in the pilot of the High School Musical spin-off series Madison High, however the pilot was not picked up to series. As of 2013, she plays Avery Jennings on the Disney Channel series Dog with a Blog, it premiered on October 12, 2012. Hannelius has several singles available on the iTunes Store.