William Peter Moseley was born on April 27, 1987, in Sheepscombe, Gloucestershire, England, to Peter and Julie Moseley. He is the eldest of three with a younger sister named Daisy and and younger brother called Ben. His father's name is also William's middle name.

Described by those who know him as a rather opinionated young man who knows his own mind, he wanted to be act since he was 10 years old. The young actor had a small role in Goodbye, Mr. Chips as Forrester, and he was cast as an extra in Cider with Rosie. However, his big break came when he was cast in the part of Peter Pevensie in The Chronicles of Narnia: The Lion, the Witch and the Wardrobe, picked out of thousands of boys for the role. He hopes he won't end up type-cast as Peter and to go on to carve a successful, well-rounded career in acting and directing in the future.

William has also just finished Year 13.
He is the eldest son of Peter, a cinematographer, and Julie Mosely. He has a younger sister Daisy (b. 1989) and a younger brother Ben (b. 1992). As a 7-year-old, he used to listen to the Chronicles of Narnia audio tapes. He read the books eight years later in preparation for his role as Peter Pevensie in the film adaptation The Chronicles of Narnia: The Lion, the Witch and the Wardrobe. He had to repeat Year 12 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 remember William when she was filling roles for Cider with Rosie back in 1998. Seven years later, when casting for The Chronicles of Narnia: The Lion, the Witch and the Wardrobe, 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 attended Sheepscombe Primary School between September 1992 to July 1998. He auditioned, unsuccessfully, for the role of Harry Potter. Because of his major growth spurt, he had to have his armor modified weekly during the filming of The Chronicles of Narnia: The Lion, the Witch and the Wardrobe. Grew 6 1/2 inches during the filming of The Chronicles of Narnia: The Lion, the Witch and the Wardrobe.