[1][3] She also was active in the British National Youth Theatre and won the Shakespeare Cup at the Kent Drama Festival.

[4] In 1963, Merrow was cast in the lead role of a BBC adaptation of Lorna Doone and subsequently had roles in British ITC TV series such as Danger Man, The Saint, The Baron, The Prisoner (in the 1967 episode "The Schizoid Man" as Alison, a mind reader), Gerry Anderson's UFO, and The Avengers where, having appeared in the penultimate episode of the 1967 series ("Mission ...

She featured in a new version of the Nigel Kneale adaptation of Nineteen Eighty-Four (1965) which was broadcast in the Theatre 625 series.

Merrow starred in the British science fiction film Night of the Big Heat (1967) with Peter Cushing and Christopher Lee, prior to her most prominent role as Alais, the mistress of Henry II (played by Peter O'Toole) in The Lion in Winter (1968), for which she received a 1969 Golden Globe nomination in the category of actress in a supporting role,[6] losing to Ruth Gordon who won for Rosemary's Baby.

Around this time, she moved to the U.S., where she guest-starred on television in dramas, mysteries and adventure programmes, including Mission: Impossible; Bearcats!