Hewson was born in Canberra and grew up in the suburb of Hughes. They attended Alfred Deakin High School and Canberra College. They began acting as part of the Canberra Youth Theatre. Hewson’s mother, Angela, is a public servant and their father, Tony, is a psychologist. Hewson has three brothers. Growing up, their family watched a lot of American television shows, such as The Simpsons and Futurama, which Liv said made it easy to act with a US accent.
Their written work includes “lots of re-imaginings of fairytales and mythology with a queer and dark slant.”