Moseley was born in the Cotswolds village of Sheepscombe, Gloucestershire, England. He is the son of Juliette E. (née Fleming) and Peter A. Moseley, a cinematographer. Moseley is the eldest of three children: Daisy Elizabeth (born 1989) and Benjamin Hugh Moseley (born 1992). Moseley attended Sheepscombe Primary School from September 1991 to July 1998, and then continued his education at Wycliffe College. He had to repeat upper sixth at Downfield Sixth Form, Stroud (which is part of Marling School and its next door neighbour Stroud High School) because he had taken so much time off school while filming The Chronicles of Narnia: The Lion, the Witch and the Wardrobe.
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.