His first book, The Snow Geese (2002), is an account of following snow geese from Texas to their summer breeding grounds on Baffin Island, and a meditation on the idea of home. It was shortlisted for the 2002 Samuel Johnson Prize and won the 2003 Hawthornden Prize, the 2003 Somerset Maugham Award and the 2003 Sunday Times Young Writer of the Year Award. His second book, The Music Room (2009), is the story of growing up in an English castle alongside a brother who suffers from severe epilepsy and brain damage. "The Music Room" has been called "a small masterpiece, a tribute to the power of place, family and memory", "sublimely evocative" and "a beautiful and fortifying book, even a great one." "The Music Room" was shortlisted for the Costa Book Awards, the Duff Cooper Prize, the Ondaatje Prize, the PEN/Ackerley Prize and the Independent Booksellers' Book of the Year Award. William Fiennes has also written for Granta, the London Review of Books, the Observer, the Daily Telegraph and the Times Literary Supplement.

Fiennes spent two years as Fellow in the Creative Arts at Wolfson College, Oxford. Since 2007, he has been Writer-in-Residence at the American School in London, and at Cranford Community College, Hounslow. He is the Director and co-founder of the charity First Story, which promotes writing in challenging schools in Great Britain.

William Fiennes is a descendent of the Fiennes family of Broughton Castle near Banbury, Oxfordshire. He is the youngest son of Nathaniel Fiennes, 21st Baron Saye and Sele (b. 1920) and Mariette née Salisbury-Jones. He is a second cousin of the explorer Sir Ranulph Fiennes, and a third cousin of the actors Ralph Fiennes and Joseph Fiennes. He is a distant relative of the writer Celia Fiennes.