William Moses was born in Los Angeles, the son of actress Marian McCargo (1932–2004) and advertising executive Richard Cantrell Moses Sr., who married in 1951 and divorced in 1963. Marian remarried in 1970 to the Republican Congressman Alphonzo E. Bell Jr. (1914–2004). Congressman Bell adopted Billy and his brothers Rick and Harry. William briefly attended Wesleyan University.
Moses is probably best known for playing Cole Gioberti in the 1980s prime-time soap opera Falcon Crest, from 1981-87. Following six seasons on Falcon Crest and some episodes in the middle of the seven season, he co-starred in the film Mystic Pizza in 1988. He then co-starred with Raymond Burr in Perry Mason television films from 1989-95, playing Ken Malansky, an attorney who worked with the title character Mason as a private investigator (although in his first appearance as Malansky, Moses played a law student who was defended by Mason on a murder charge). Moses returned to soap opera in 1992–93 when he played the recurring role of Keith Gray on Melrose Place. He made appearances in two Adventures of Mary-Kate and Ashley short films, The Case of the Mystery Cruise and The Case of the Sea World Adventure, both in 1995.
In 1997, he played David Graysmark in the short-lived series Fame L.A. He also appeared in various television movies including Alone With a Stranger (2000), and Christmas Child (2003). Since 2005, he has appeared as Jack Davis in nine of the Jane Doe movies on the Hallmark Channel. In recent years, he has appeared in various movies on the Lifetime channel, including A Lover's Revenge (starring his former Perry Mason co-star Alexandra Paul), The Perfect Marriage (co-starring Jamie Luner and James Wilder), and Killing Daddy (starring Elizabeth Gillies). Throughout his career, Moses has made guest appearances on a wide variety of television series, including Fantasy Island, The Love Boat, Hotel, Murder, She Wrote, Ally McBeal, JAG, NCIS, 7th Heaven, Touched by an Angel, Cold Case, CSI: Miami, Without a Trace, Ghost Whisperer, Castle, The Mentalist and Bones. He also appeared on The Secret Life of the American Teenager and season 5 of Homeland.