Newsom was born and raised in the small town of Nevada City, California. Her mother, Christine (née Mueller), is an internist, and her father, William Newsom, is also a doctor. Her parents were “progressive-minded professionals” who previously lived in the Bay Area. Newsom’s family includes her brother, Pete, a fellow musician, and sister, Emily, who inspired her song “Emily” (and contributed backing vocals). She is the second cousin, twice removed, of Lieutenant Governor Gavin Newsom of California.
As a child, Newsom was not allowed to watch television or listen to the radio because she was raised by parents who she described as, “kind of idealists when it came to hoping they could protect us from bad influences, like violent movies, or stupid stuff”. She was exposed to music from a young age. Her father played the guitar and her mother was a classically trained pianist who played the hammered dulcimer, the autoharp and conga drums. Newsom attended a Waldorf school where she studied theater and learned to memorize and recite long poems. This skill helps her to remember lyrics while on tour.
At the age of five, Newsom asked her parents if she could play the harp. Her parents eventually agreed to sign her up for harp lessons, but the local harp instructor did not want to take on such a young student and suggested she learn to play the piano first. Starting at the age of four she began playing the piano. Only later did she move on to the harp, which she, “loved from the first lesson onward.” From her instructor, Joanna learned composition and improvisation. She first played on a smaller Celtic harps until her parents bought her a full-size pedal harp in the seventh grade. During her teens, she and the instrument became inseparable, and she describes her relationship with the harp as similar to “an artificial limb or a wheelchair. It’s almost part of me, but more to the point, it serves a purpose, and if it wasn’t there I would wonder what was supposed to fit in its place.”