Jemma belongs to a theatrical dynasty going back to Victorian times.
Her father is the actor Corin Redgrave and her mother, Deirdre Hamilton-Hill, was the daughter of an officer in the Royal Navy. Her parents divorced when Jemma was nine, and her mother died in 1997 of breast cancer, after writing a book about the failed marriage, To Be a Redgrave.
Jemma trained at the London Academy of Music and Dramatic Art. She is married to Tim Owen Q.C., a London barrister, and they have two sons, Gabriel (born 1994) and Alfie (2000). During a separation from Tim during the late 1990s, Jemma had an affair with the actor Jonathan Cake, but she made it up with Tim and they soon had their second child.
In 2002-03, Jemma was an active member of the "Don't Attack Iraq" movement, campaigning against the invasion of Iraq by American and British forces.