Mirren attended St. Bernard's High School for Girls, a convent school in Southend-on-Sea. It was here that her interest in acting began to develop, and she participated in school productions.

At the age of 18, Mirren successfully auditioned for the National Youth Theatre (NYT). This marked a pivotal moment, providing her with invaluable experience and exposure in a professional theatrical environment. The NYT was a breeding ground for young talent and gave Mirren a solid foundation.

Despite her success with the NYT, Mirren auditioned for drama schools. She was accepted into Central School of Speech and Drama, but chose to pursue her career with the Royal Shakespeare Company instead.