Hansen grew up in Santa Ana, California. She is the eldest of 8 children and grew up in a 4-bedroom-house along with 23 other people. She is of Polynesian (mainly Tongan) descent, and also has some Samoan, Fijian, and Danish roots. Hansen performed in public for the first time while singing the national anthem at the age of seven. In 2011, she recorded her first single titled "Dancing Like a White Girl". She attended Orange County School of the Arts and graduated in 2015.

Hansen auditioned for The X Factor in 2012 singing "If I Were a Boy" by Beyoncé. For her bootcamp solo performance she sang Hero by Mariah Carey and is the only member from Fifth Harmony to have it aired. She sang against Diamond White for her bootcamp round singing "Stronger (What Doesn't Kill You)" during which she forgot some of the lyrics. She was eliminated during the bootcamp round of competition, but was brought back along with Ally Brooke, Normani, Lauren Jauregui, and Camila Cabello to form the group now known as Fifth Harmony. The group made it all the way to the finals and landed in third place.

In 2015, Hansen auditioned for the role of the main character in the then-upcoming animated film Moana; the role eventually went to Auli'i Cravalho. Fuse's website included Hansen on an article of the faces of Future Asian and Pacific history, naming the most promising entertainers of the region in 2017. She was featured in RedOne's song "Boom Boom" along with Daddy Yankee and French Montana. She sang the Tongan national anthem "Ko e fasi 'o e tu'i 'o e 'Otu Tonga" on November 24, 2017 at the 2017 Rugby League World Cup semi-final game. Hansen released a Christmas medley with singer Leona Lewis.