Hayley Elizabeth Atwell was born in London, England, to Allison Cain, a motivational speaker, and Grant Atwell, a photographer. Growing up in London, she was exposed to a diverse cultural environment that fostered her interest in the arts and performance. Her dual citizenship (British and American) provided her with opportunities that broadened her perspective from a young age.

Atwell honed her acting skills at the prestigious Guildhall School of Music and Drama in London. This period was crucial in developing her foundational acting techniques, stage presence, and versatility. Guildhall provided her with rigorous training in classical and contemporary theatre, preparing her for the diverse roles she would later undertake.