Duff initially thought about pursuing a career as a singer and talked about it in great depth with her teacher, who looked at her and said, 'I think you have the soul of an actor.

After further study of Film and Theatre, at the age of 19, she attended the Drama Centre in London, alongside John Simm, Anastasia Hille and her good friend Paul Bettany.

She later made appearances in series such as Amongst Women, in Aristocrats as Lady Louisa Lennox and in 2003 BBC television film Charles II: The Power and the Passion as Henrietta of England.

Duff played Holly in the first series of Simon Nye sitcom, Wild West, alongside Dawn French and Catherine Tate in 2002.

In 2009, Duff received further attention when she played the mother of John Lennon, Julia Stanley, a role for which she won British Independent Film Award for Best Supporting Actress in Nowhere Boy.

Throughout this time, Duff continued to appear on mainstream television in Parade's End, a five-part BBC/HBO/VRT television serial adapted from the tetralogy of eponymous novels (1924–1928) by Ford Madox Ford as Edith Duchemin and in BBC One crime drama From Darkness which premiered in October 2015, appearing in the starring role.

"[8] In 2015, she played Violet Miller in the film Suffragette, a working-woman who introduces Maud Watts (Carey Mulligan) to the fight for women's rights in east London.

In 2020, Duff portrayed Erin Wiley, the estranged heroin addict mother of established character Maeve in the second season of the Netflix original series Sex Education.

In June 2020, Duff appeared in a main role as Tracy Daszkiewicz in three-part drama The Salisbury Poisonings.

Credits at the National Theatre include Collected Stories, King Lear and the title character in Marianne Elliott's production of George Bernard Shaw's Saint Joan to great acclaim.