Purnell was born in London. Up until May 2018, she lived there with her mother. However, in August 2018 she moved to Brooklyn, New York. She has three younger brothers, Noa, Enzo and Paul. She attended Forest School, formerly attended City of London School for Girls and began Year 10 in September 2011. She used to attend weekly classes at Sylvia Young Theatre School, studying acting, singing and dance, and was formerly represented by their in-house talent agency, Young 'Uns (now known as Sylvia Young Agency). She is now however represented by The Rights House. She also attended Young Actors' Theatre Islington.