Williams 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 That Girl and The Mod Squad before being cast as Greg Brady in The Brady Bunch sitcom in 1969. As the eldest of the kids, his storylines often involved his 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 theatre, 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.

In 1992 his autobiography Growing Up Brady spent three months on the New York Times bestseller list.

Always proud of his success with The Brady Bunch, Williams has appeared in various Brady TV movie reunions, has built a cabaret act that pays tribute to his past, and has played himself in Dickie Roberts: Former Child Star (2003). In 2000, when Eminem had a hit with 'The Real Slim Shady', Williams released his own parody version, 'The Real Greg Brady'.

Williams also featured as himself in the video to the Peter Gabriel song The Barry Williams Show.

He currently hosts The Brady Brunch with Barry Williams Sundays through Fridays from 2 p.m. to 6 p.m. Eastern on Sirius Satellite Radio's "Totally '70s" channel.