Atwell holds dual citizenship of the United Kingdom and the United States.

Atwell was reportedly in a relationship with musician Gabriel Bisset-Smith. The relationship lasted around two years before ending.

Atwell was in a relationship with musician Evan Jones. They were seen together at various events, but the relationship ended after about two years.

There were rumors of dating Tom Cruise.

Atwell began a relationship with Ned Wolfgang Kelly, a self-described writer, composer, and artist. The relationship was confirmed in 2022.

Atwell married Ned Wolfgang Kelly in 2023 in Venice. It was a private ceremony with close friends and family.