Merchant subsequently performed in many stage productions and several films, including Alfie (1966), Accident (1967), Frenzy (1972), and The Offence (also 1972).

After Merchant married the playwright Harold Pinter in 1956, she appeared in many of his plays, including the 1960 revival of his first play, The Room at the Hampstead Theatre, A Slight Ache, A Night Out, The Collection, and The Lover; the last was also a celebrated television production partnering Alan Badel at Associated Rediffusion, for which she was given an Evening Standard Theatre Award for Best Newcomer and the BAFTA Award for Best Actress, both in 1963.

She starred as Ruth in The Homecoming (1964) on stage in both London in 1965 and New York in 1967, receiving a Tony Award nomination for Best Actress in a Play.

Merchant took the role of Madame in the Greenwich Theatre revival of Jean Genet's The Maids partnering Glenda Jackson and Susannah York: This was filmed in 1974 by Christopher Miles.

[5] At first, Merchant took it very well, saying positive things about Fraser, according to her friend artist Guy Vaesen (as cited by Billington); but, Vaesen recalled, after "a female friend of Vivien's trotted round to her house and poisoned her mind against Antonia ... life in Hanover Terrace [where the Pinters then lived] gradually became impossible".

Pinter left, and Merchant filed for divorce and gave interviews to the tabloid press, expressing her distress.

"[8] Merchant believed Fraser to be the basis for the character of Emma in Pinter's play Betrayal, never learning about his prior affair with Joan Bakewell.