Williamson was an Auror. He was described as wearing red robes and a long pony tail when Harry Potter first met him. He was the first Auror to see Lord Voldemort just before he fled the Ministry of Magic in 1996 after the Battle of the Department of Mysteries. Fudge then sent him and John Dawlish to deal with the captured Death Eaters.
- Harry Potter and the Order of the Phoenix (First appearance)