Millie was in jail in The Housemaid because, as a teenager, she killed a boy while defending her best friend from an attempted rape; it is portrayed as an accidental killing in the context of self-defense.

What actually happened

  • Millie was at a boarding school when a boy tried to sexually assault her best friend, Kelsey.
  • In the struggle to protect Kelsey, Millie beat the boy, and he ended up dying from his injuries, which led to her being sentenced to prison.
  • The narrative frames this as a morally complex but largely protective act, emphasizing that her violence came in response to an attempted rape, not random aggression.

How this ties into The Housemaid plot

  • Nina specifically chooses Millie as a housemaid after discovering this prison record, believing Millie might be capable of killing Andrew, something Nina herself feels unable to do.
  • Millie’s past explains both why she seems “dangerous” on paper and why she is unusually resilient when Andrew begins his psychological and physical abuse.
  • This backstory becomes a key twist: what looks like a simple “ex-con housemaid” setup is actually part of Nina’s long, calculated plan to turn Millie into Andrew’s replacement victim—and potential executioner.

TL;DR: Millie went to prison because she fatally injured a boy who was trying to rape her friend; later, Nina exploits that history in her scheme against Andrew in The Housemaid.

Information gathered from public forums or data available on the internet and portrayed here.