Ashley Nicolette Frangipane was born in Edison, New Jersey, on September 29, 1994,^[9] the daughter of Nicole and Chris Frangipane. Her parents dropped out of college after her mother discovered that she was pregnant with her.^[10] Halsey's mother now works as an emergency medical technician (EMT),^[11] while her father manages a car dealership.^[12][13] Her mother is of Italian and Hungarian descent, whereas her father is mostly African-American with some distant Irish ancestry. She has two younger brothers named Sevian and Dante.^[10] Halsey played the violin, viola, and cello until moving on to the guitar when she was 14 years old.^[14] She grew up listening to Alanis Morissette, Justin Bieber, and Brand New.^[15][16] Throughout her childhood, her family moved frequently, as her parents worked many jobs. By the time she reached her teen years, she had enrolled in six schools.^[17]

Frangipane was bullied at school,^[13][18][19] and her suicide attempt at the age of 17 led to her being hospitalized for nearly three weeks. Following this, she was diagnosed with bipolar disorder, with which her mother was also diagnosed.^[20][21] She began using drugs soon after, saying her bipolar disorder caused her to become an "unconventional child".^[17] Also when she was 17, she became romantically involved with a man who was 24 and resided on Halsey Street in Brooklyn, New York, from where she took her stage name. She said, "That's where I first start[ed] writing music and where I started to feel like I was a part of something bigger than my town in middle of nowhere New Jersey. Halsey is kind of like a manifestation of all the exaggerated parts of me, so it's like an alter ego."^[22] In 2012, she graduated from Warren Hills Regional High School in Washington, New Jersey.^[23]

After graduating, Frangipane enrolled in the Rhode Island School of Design, but she withdrew due to financial hardship and instead attended community college.^[10] She eventually dropped out of community college and was kicked out by her parents, who she said "just didn't agree with a lot of things about [her]". Soon after, she lived in a basement in lower Manhattan with a group of "degenerate stoners" she knew through her then-boyfriend.^[24] When she was not living there, she occasionally lived in one of New York's many homeless shelters, and she considered prostitution as a way to make money.^[25][26] When describing this period of her life, she has said, "I remember one time I had $9 in my bank account, and bought a four-pack of Red Bull and used it to stay up overnight over the course of two or three days, because it was less dangerous to not sleep than it was to sleep somewhere random and maybe get raped or kidnapped." She would occasionally stay with her maternal grandmother during this time.^[10]