Barry Williams was born in Santa Monica, California to Doris May Moore and Canadian-born Frank Millar Blenkhorn of English, Scottish and German ancestry. He decided as a very young child that he wanted to be an actor, and in 1967 he made his television debut in an episode of Dragnet. He played guest roles in Adam-12, The Invaders, That Girl, Mission: Impossible and The Mod Squad before being cast as Greg Brady in The Brady Bunch sitcom in 1969. As the eldest of the kids, his story lines often involved his many romances, and as the Greg character reached his mid-teens the show's producers began to groom and promote Williams as a teen idol.

With the show's demise in 1974, Williams was unable to attract the type of high profile roles that might have expanded on his earlier success. He resumed playing guest roles on television, and became involved in musical theater, touring with productions such as Grease, The Sound of Music and West Side Story, as well as making unsuccessful attempts to establish a career as a recording artist. However he was later able to capitalize on being type-cast as "Greg Brady." Amid a procession of appearances in TV and movies that played up his famous teen role, Williams ended up landing a role that was quite a departure from the Brady image, and that could have shaken off that image if the role was held longer: briefly in 1984, he was tapped to play English con man Hannibal, who conspired with Holly Sutton Scorpio (Emma Samms) on the top-rated General Hospital. Although the appearance was well-received, Williams was not asked back after his initial story arc was concluded.

His 1992 autobiography, Growing Up Brady: I Was a Teenage Greg, written with Chris Kreski, spent three months on the New York Times bestseller list. The book was adapted into a 2000 TV movie titled Growing Up Brady that starred Adam Brody as Williams.

Always proud of his success with The Brady Bunch, Williams has appeared in various Brady TV movie reunions, including the hit 1988 Christmas movie "A Very Brady Christmas" where his sole family problem is missing his wife, Nora. His problem is solved when Nora shows up at the Brady house on Christmas with a poinsettia in hand. He also has built a cabaret act that pays tribute to his past, and has played himself in the film Dickie Roberts: Former Child Star (2003), which is about a former child star who finds himself out of work. In 2000, when Eminem had a hit with "The Real Slim Shady," Williams released his own parody version, "The Real Greg Brady."

In 2001-2002, Barry played Manager Dean "The Machine" Strickland in 13 episodes of the TV sitcom S Club 7 in Hollywood which featured the British pop group S Club 7.

Williams appeared briefly as an audience member in the music video of Peter Gabriel's song "The Barry Williams Show" (2002), which is not about the actor. Gabriel said he hadn't known about Williams when he wrote the song.

Williams appeared in the 2002 song Oakland Raiders by San Francisco rap group Luniz. Delivering a humorous monologue as the song faded out, Williams asserted his status as a pimp and stated his fondness for big dank.

Williams participated in a Celebrity Boxing match with former Partridge Family TV star Danny Bonaduce, but lost to Bonaduce, who is a martial arts black belt.

Barry also appeared with his former Brady Bunch co-star Christopher Knight (Peter Brady) on a 2006 episode of the Fox network sitcom That '70s Show. He and Knight played a gay couple who moved in next door to the Formans. He and Knight have remained close friends since their Brady Bunch days, and he appeared in several episodes of Knight's reality show series My Fair Brady.

Barry Williams currently hosts The Real Greg Brady’s Totally '70s Pop Quiz starring Barry Williams Saturdays from 8 p.m. to midnight Eastern Time on Sirius Satellite Radio's "Totally '70s" channel.