Dennis earned a Master of Fine Arts degree from The American Film Institute Conservatory. The Oz project was Dennis's first screenwriting credit in Hollywood. He has scripted other projects since, like the 2005 remake of Walk the Line. He directed the 1995 feature Without Evidence and co-wrote its screenplay. He also has acted in several films and television shows.
Dennis was married to the actress Elizabeth Hartman (d. 1987) from 1968 to 1984.