Casting director Pippa Hall was responsible for casting William as an extra when she was filling roles for Cider with Rosie (TV) back in 1998. Seven years later, when casting for The Chronicles of Narnia: The Lion, the Witch and the Wardrobe in 2005, she remembered him and recommended him for the role of Peter. From the first day of auditioning for the role of Peter Pevensie in The Chronicles of Narnia: The Lion, the Witch and the Wardrobe it took a further eighteen months before he officially won the role and was cast. He reprised the role of Peter in the 2008 sequel, The Chronicles of Narnia: Prince Caspian. Moseley made a cameo appearance as Peter in The Chronicles of Narnia: The Voyage of the Dawn Treader despite his character's absence in the book on which the film is based. Moseley recently portrayed Aysel in upcoming Brent Ryan Green film The Veil.
William Moseley was born in the Cotswolds village of Sheepscombe, Gloucestershire, England. He is the son of Juliette (née Fleming) and Peter Moseley, a cinematographer. Moseley is the eldest of three children: Daisy (born 1989) and Benjamin Moseley (born 1992). Moseley attended Sheepscombe Primary School from September 1991 to July 1998, and then continued his education at Wycliffe College with final year at Downfield Sixth Form, Stroud (which is part of Marling School and its next door neighbour Stroud High School). He had to repeat upper sixth because he had taken so much time off school while filming The Chronicles of Narnia: The Lion, the Witch and the Wardrobe.
- He has joined the Stand Up to Cancer programme "Change the Odds" featuring many other young actors.