Hannelius was born in Boston, Massachusetts. She moved to southern Maine at age 1. 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.