Born in London, Dunn developed an interest in filmmaking at an early age, making films as a teenager before studying film at what is now the University of Westminster. He began his professional career at the BBC, where he worked his way through the industry and earned recognition for his cinematography on landmark television productions including Threads, Edge of Darkness and Tumbledown, receiving three BAFTA Awards for his work.
Dunn transitioned successfully into feature filmmaking, building a diverse filmography that includes L.A. Story, The Bodyguard, The Madness of King George, The Crucible, Gosford Park, The History Boys, Precious, Crazy, Stupid, Love, The Perks of Being a Wallflower, Lee Daniels' The Butler, The Children Act, Downton Abbey: A New Era and The United States vs. Billie Holiday. Throughout his career he has worked with directors including Robert Altman, Nicholas Hytner, Mick Jackson, Richard Eyre and Lee Daniels.
In addition to his BAFTA honours, Dunn has received the British Society of Cinematographers Award and the Evening Standard British Film Award for his contribution to The Madness of King George. His work has been recognised by both the British and international film industries, and he remains a respected figure in contemporary cinematography.