At the age of 12, Williams started her professional acting career by co-starring in one of the largest ensemble casts on television. She was cast in the role of Arya Stark, the feisty young tomboy daughter of an important noble family in HBO's historical fantasy drama series "Game of Thrones" (based on George R. R. Martin's "A Song of Ice and Fire" epic fantasy novel series). Williams had almost missed the audition (her second ever), since it coincided with a school trip; her mother convinced her to go to the audition. As the series viewership rose, the international popularity of "Game of Thrones" gave Williams global recognition.