Vanessa Nuala Kirby was born in Wimbledon, London, to Jane (née Searby), founder of a magazine, and Roger Kirby, a urologist. The exact year of her birth is sometimes debated between 1987 and 1988, but most sources indicate 1987.

Kirby grew up in a household that valued creativity and intellectual pursuits. Her mother's work in magazine publishing and her father's profession exposed her to different perspectives and likely nurtured her interest in storytelling.

Kirby attended Lady Eleanor Holles School in Hampton, London, a prestigious independent day school for girls. This provided her with a strong academic foundation and likely opportunities to participate in extracurricular activities, though specific details are scarce.

After completing her secondary education, Kirby took a gap year during which she travelled to South Africa and worked with a charity and participated in theatre projects. This experience solidified her passion for acting and convinced her to pursue it professionally rather than attending university immediately.

Despite her strong desire to study acting, Kirby was initially rejected from the Bristol Old Vic Theatre School, a highly regarded drama school. This early setback likely motivated her to work even harder to hone her skills.

Kirby eventually decided to pursue higher education, graduating with a BA in English from the University of Exeter. While not a formal acting program, her English studies likely honed her understanding of literature and storytelling, skills valuable to an actor.