Born in Indianapolis, IN he graduated from Shortridge High School. He studied journalism at Butler University. He published his first novel in 1979 and went on to publish more than 40 novels and 150 short stories. In 2003 he received a lifetime achievement award from the Horror Writers of America. He edited the critically acclaimed How to Write Tales of Horror, Fantasy & Science Fiction (1987) which covered the themes of such writing and cited the writings of such writers as Robert Bloch, Lee Prosser, Richard Matheson, Ray Bradbury, H. P. Lovecraft, August Derleth, William F. Nolan, and Stephen King. Many important writers in the genre contributed to the book. Williamson edited the popular anthology series, Masques. Some of his novels include The Ritual (1979), Playmates (1982), Noonspell (1991), The Haunt (1999), among others.
He was also a well known Sherlockian and received his investiture (The Illustrious Client) in the Baker Street Irregulars in 1950.