He and his wife Beverly moved to Los Angeles in 1966, where for many years, they were active in LASFS. As an assistant to Forrest J Ackerman, Warren came into contact with many major filmmakers-in-waiting who were also inspired by Ackerman, and developed independent friendships with several of them.

His 1968 short story "Death Is a Lonely Place" appeared in the first issue of Worlds of Fantasy, and he also scripted for the Warren comic book publications, Creepy, Eerie and Vampirella.

In the late 1960s and early 1970s, Warren made contributions to Walt Lee's Reference Guide to Fantastic Films (Chelsea-Lee Books, 1972); Bill did the last small bits of research, helped plan the project and typed the entire thing. He is a regular contributor to Leonard Maltin's Movie Guide, a book series of movie capsule reviews that began in 1969 and was updated annually beginning in 1978.

The first volume of Warren's authoritative and exhaustive Keep Watching the Skies! American Science Fiction Movies of the Fifties was published by McFarland in 1982, and he followed with the second volume in 1986. This massive survey of science fiction films released from 1950 to 1962 was reprinted in 1997. A completely revised edition was released in 2009.

Warren was the film critic for a newspaper in Simi Valley, California during the 1980s. In 1989, he created the ShowBiz Roundtable for the online service GEnie to generate discussions about movies of all sorts. Following the traditions of newspapers, he only posted his GEnie reviews online the day a film opened. After these reviews were posted, they were archived in a GEnie Library. With weekly postings during the early 1990s, this became the first large film review database available online. Warren also conducted live online interviews with film personalities on GEnie.

At the request of film director Sam Raimi, Warren wrote The Evil Dead Companion in 1994, but six years passed before the book was published in the UK by Titan Books (2000), followed by an American edition from St. Martin's in 2001. Warren found his friendship with Raimi a life-altering experience:

I started with Sam on Darkman. He did something that actually changed my life. At the time, he was living not far from where I do, in Silver Lake; this was before he was married, and he lived in a little cabin. I went over to interview him and Scott Spiegel, and they were blown away when I said that I used to know Fritz Lang. They said, "OK, who else did you know?" I said Boris Karloff, and they just went out of their minds. At that time, I had been feeling very sorry for myself, and Sam said, "Gee, you must've met everybody you've ever wanted to meet." When I went home I started thinking about it. And I realized I indeed had met everybody I ever really wanted to meet, with the exception of three people; one of those, Vincent Price, I met later that year, and the other two, one of whom was Peter Cushing, never had a chance to meet. But I suddenly realized my life wasn't as bad as I thought it was, and it was because of Sam's remarks. He changed my way of thinking at that point, and it's never changed back, and I have Sam to thank for that. I was going to put that in the book, but someone said it's too personal, so it's not in the book. Sam has read the book and he likes it.

Warren occasionally contributes to supplemental material for DVDs. During the 1990s, he teamed with film director William Rotsler to produce segments surveying American television for the French TV series Destination séries.