Gabbard was born in American Samoa in 1981. Her father was a Vietnam War veteran and her mother is a nurse. Gabbard moved to Hawaii when she was two years old. She attended Punahou School, the same school that President Barack Obama attended. Gabbard went on to study political science at the University of Hawaii at Manoa. After graduating from college, Gabbard worked as a legislative aide to Democratic Congressman Neil Abercrombie.
Gabbard was elected to the Hawaii House of Representatives in 2002. She served in the state House for four years, during which time she was a vocal advocate for environmental protection and renewable energy. In 2006, Gabbard was elected to the Honolulu City Council. She served on the City Council for four years, during which time she was a strong advocate for affordable housing and public transportation.