Born in London, Williams trained at the Central School of Speech and Drama there, graduating in 1994. She has appeared in a number of British films, nearly always alongside her mother. Her picture appears in the opening credits of As Time Goes By (1992).
Williams has one child, a son called Sam, who was born on 6 June 1997. Her mother said Williams didn't tell her she was going to be a mother until only days before the birth.