William Marsey is a UK-based composer.

He has worked with the LA Phil, BBC Arts, and the Royal Northern Sinfonia, and his music has been performed around the UK as well as in Canada, China, and the USA. He has been nominated for a British Composer Award, and his work has been broadcast on BBC Radio 3, Radio 5 Live and the World Service. His collaborations have been exhibited in the UK, Zurich, Paris, and Austin Texas. He has worked with musicians including Oliver Knussen, Liam Byrne, Huw Watkins, Zoe Martlew, Lawrence Power, and Thomas Adès.

He is co-founder and artistic director of Listenpony, a charity, concert series, commissioning body, and record label for new and old and pop music. To date the company has produced 24 concerts, 7 commercial recordings, and commissioned over 50 new works.

Born in Hartlepool, County Durham, William studied composition with Giles Swayne at the University of Cambridge, with Gary Carpenter and Edmund Finnis at the Academy, where he was also Manson Fellow, and Laurence Crane and Richard Baker at the Guildhall School of Music and Drama. He lives in London.