William was born in 1948, and spent much of his early life working with Assassin cells around the world. By the 1980s, he settled in a compound known as "The Farm", where he trained recruits, including Clay Kaczmarek, Lucy Stillman, and his own son, Desmond.
In 2003, Desmond rebelled and ran away. He resurfaced in 2012, when he was abducted by Abstergo and forced into the Animus Project, attempting to locate an Apple of Eden. William led the rescue effort and brought Desmond to New York. Father and son, along with Shaun Hastings and Rebecca Crane, found a First Civilization temple, and worked together to prevent a prophesied global catastrophe. Saving the world came at a high cost: Desmond died, and the Precursor entity known as Juno was released.
Grieving, William retreated from the Assassins, handing off leadership to his friend Gavin Banks, along with a coded journal that contained many of his secrets. One year later, William has returned to the fight, but his motivations remain elusive as ever.