Byron was born Kathleen Elizabeth Fell in Manor Park (then part of Essex)[1][2] to what she described as "staunch working-class socialists", who later became Labour mayors of the County Borough of East Ham.

She had her first speaking film role in Carol Reed's The Young Mr. Pitt (1942), in which she had two lines as a maid opposite Robert Donat.

She had an occasional role in the 1957–67 soap Emergency Ward 10, playing the alcoholic wife of the consultant gynaecologist Harold de la Roux (John Barron).

In the 1960s and 1970s, Byron did extensive television work, including a 1961 appearance in a Danger Man episode entitled "Name, Date and Place" as Deidre; Crown Court (episode: "A Case of Murder"); a small role as Queen Louise of Denmark in Edward the Seventh (1975), Madame Celeste Lekeu in two episodes of the BBC drama Secret Army (1977), entitled "Bait" and "Good Friday", a brief stint on the soap opera Emmerdale Farm in 1979, and one of the leads in the daytime soap Together (1980–81, its second series broadcast live).

Byron continued to act into the new millennium, her film, theatre and television work including Agatha Christie's The Mousetrap (1990), an adaptation of Jane Austen's Emma (1996), Steven Spielberg's Saving Private Ryan (1998), Midsomer Murders (1999) (as Dorothea Pike in S2:E2 “Strangler’s Wood”) and Stephen Poliakoff's series, Perfect Strangers (2001).