John-Kamen was born in Anlaby, East Yorkshire, the daughter of a Nigerian forensic psychologist and a Norwegian fashion model.
She attended primary school in Kirk Ella and received her secondary education at Hull Collegiate School, and also trained at the National Youth Theater in London.In 2012, she graduated from the Central School of Speech and Drama.
John-Kamen began her professional career in 2011 when she provided her voice for the video game Dark Souls. She then went on to make episode appearances in television series Misfits (2011), Black Mirror (2011), Whitechapel (2012), The Syndicate (2012), The Midnight Beast (2012) and The Hour (2012).
In 2012, John-Kamen landed the lead role of Viva in Viva Forever, a West End musical based on the songs of the Spice Girls. Written by Jennifer Saunders and produced by Judy Craymer, Viva Forever premiered on 11 December 2012 at the Piccadilly Theater to largely negative reviews. The Daily Mirror, however, praised John-Kamen's performance, noting, "It’s a shame a talented cast, especially Hannah John-Kamen’s Viva and the rest of Eternity, are let down by a clichéd plot and leaden dialogue." The show was eventually closed on 29 June 2013.
Hannah John-Kamen is currently single and weighs 350 pounds.