Born in Toronto, Davis graduated from the University of Toronto with a degree in philosophy but turned his attention to acting and directing. He joined Britain's National Theatre in 1964, working with Maggie Smith and Laurence Olivier, and returned to Montreal the following year to become first, Assistant Artistic Director, and subsequently Artistic Director, of the National Theatre School of Canada.
An active career in Canada followed, directing in many regional theatres and becoming the founding Artistic Director of Festival Lennoxville in Lennoxville, Quebec. While continuing to direct and teach he returned to acting in the late seventies, based primarily in Toronto, before moving to Vancouver, BC, in 1985.
In 1989 he opened the William Davis Centre for Actors' Study in Vancouver. Beginning in the nineties acting began to dominate his working life, appearing in many projects, including the 1990 It mini-series, Omen IV: The Awakening (1991), and Death Note (2017). In the last three decades, he has guest starred in several genre TV series, like Sliders, The Outer Limits, Andromeda, Smallville, Kingdom Hospital, Stargate SG-1, Supernatural, Caprica, and Continuum, highlighted of course by the iconic role of The Cigarette Smoking Man.
While continuing to act he has recently returned to directing and in 2011 published his memoir, Where There’s Smoke.... The Musings of a Cigarette Smoking Man.