Born Micheaux, Bill was a follower of Rambaldi, who got involved in decryption work for the organisation Prophet Five. In 1972 they gave portions of the code to a number of different individuals, but when the work was nearly completed, Bill realised that Prophet Five were killing the others. Managing to warn fellow targets James Lehman and Luc Goursaud, they fled with their families and changed their names, although Michael Vaughn was too young to remember this. Bill then joined the CIA, but was killed by KGB agent Irina Derevko in 1979, when Michael was 8 years old.
Thomas Brill told Michael Vaughn that Bill was a follower of Rambaldi and that he was with Bill on his final mission, to rescue a little girl "The Passenger". According to Thomas Brill, in 1985/86 Bill rescued a young Nadia Santos from a KGB compound where she spent the first few years of her life as the subject of various experiments. Bill kidnapped Nadia from Soviet custody and left her at an orphanage with Sophia Vargas aka Elena Derevko.
When visiting his dying, comatose uncle, Michael Vaughn comes into possession of a journal in his father's handwriting, with entries three years after his supposed death. His investigation put him in contact with someone who claimed that his father was still alive, and that they could provide his location. Confiding in Sydney Bristow, Michael Vaughn stole the object of an APO mission for his mystery contact, going AWOL from APO. He took part in a mission they had organised to steal an artifact from a CIA convoy, but when he realised they'd lied about his father being alive, turned on them, bringing both objects back to APO. It's safe to assume that Thomas Brill was working for Elena Derevko as a ploy to get hold of a Rambaldi device,and that Bill was dead all along. He was killed by Irina Derevko in 1979.