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 Madeline in Madeline's Rescue 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 3 months to try out auditions, and she was so successful that her family relocated permanently to Los Angeles in 2008.
Hannelius played Courtney Patterson on Surviving Suburbia. She had a recurring role on Sonny with a Chance, a Disney Channel Original Series as Dakota Condor, a guest role on Hannah Montana as a fan named Tiffany and she had a recurring role on Good Luck Charlie as Jo Keener. Hannelius is an alumna of the Los Angeles-based Young Actor's Studio. She played Madeline in the Children's Theater of Maine's production of Madeline's Rescue in summer 2002. She also played Emily Pearson in Den Brother in 2010, and was the voice talent of Rosebud in the new 2011 Spooky Buddies. In February 2011, she was cast for a lead role in the pilot of the High School Musical spin-off series Madison High. She plays Avery Jennings (lead role) on the new Disney Channel show Dog With a Blog that premiered on October 12, 2012. In 2013 Hannelius appeared in one episode of Fish Hooks as the voice of Amanda. Hannelius has five singles out on iTunes, 'Staying Up All Night', 'Two In A Billion', 'Just Watch Me', 'Paper Cut' and 'Sun In My Hand'. Hannelius is not signed.