He was born William Allen Thornbury on May 24, 1952 in Kingsburg, California. He started acting professionally in 1974 at the age of 22 by playing in an independent film entitled Summer School Teachers. He also directed Barbara Peeters.

After such a youthful start to his career, he landed parts in TV shows and movies. One was the 1975 version of Sarah T, Portrait of a Teenage Alcoholic. Also around this time he appeared in an episode of the then very popular The Rockford Files with James Garner. During this period of his life he also accepted roles in several national television commercials. These included working for such notable companies as Honda Motorcycles, Coca-Cola and Chrysler/Plymouth.

His most high profile role, however, came in 1978 at the age of 26. The movie Phantasm was a low budget horror film by writer and director Don Coscarelli. Bill played the part of Jody Pearson, the older brother of the main character Mike Pearson. The original filming and production took over two years. During this time, Bill became great friends with Don as well as the other actors in the movie: Reggie Bannister, A. Michael Baldwin and Angus Scrimm. Their friendship still lasts today and they have been referred to by their fans as The Phantasm Phamily.

Despite being a low-budget movie, Phantasm has picked up some high-profile fans including a motivational speaker who had this to say about the movie:

"The simple tag line: "If this doesn't scare you, you're already dead," stayed with me for decades and helped me appreciate how powerful a single sentence can be."

Because of Bill's talent as a musician, singer and songwriter he was asked to compose one of the theme's of the movie. It is a country song called Sittin' Here at Midnight and is one of the most memorable moments in the movie. He sang the piece along with fellow actor Reggie Bannister. It is now a collector's item for fans of the movie world over.

After Phantasm, Bill chose to pursue his music as a career and did not do much acting after that. He did, however, do two roles during the 1980s. One of the parts was in a daytime drama entitled Secrets of Midland Heights and one was in The Lost Empire. At this point he retired from acting entirely in order to focus on performing and teaching music.

He is a professional singer/songwriter for 20th Century-Fox and MTM Records. He also performs in the band The Sons of the San Joaquin. He lives in Northern California with his actress wife, Sharon Thompson and their son, Ben. She also teaches music near their home in California when she is not busy acting.

One of the things Bill enjoys is leading the Blue Jean Worship at the Fresno First Presbyterian Church. He also teaches music full-time at the St. Helen's School of the Performing Arts and Sciences. And he's still writing and performing music himself, having recently released a CD called 'Torn Little Page'. In 2010 Bill wrote twenty-two songs for the musical Rancho Tesoro. The show was written by Brian Roberts and premiered at the L.J. Williams Theatre in Visalia, California.