Johnston was best known for her stage performances, but also appeared in 12 films and a handful of TV productions before retiring from acting in 1968 to devote herself to running a theatrical agency.

[1] Johnston had shown an interest and aptitude for drama from an early age acting in various school productions before working professionally in Sydney's theatres.

Critic Alan Dent noted that "the one exception among the general cowed lethargy of the Barretts is Elizabeth herself...she is sincere, touching, awakened.

"[3] Johnston appeared as Alma Winemiller in a 1951 production of Tennessee Williams' Summer and Smoke, initially at the Lyric then transferring to the Duchess Theatre.

Johnston made her screen debut in the film The Prime Minister (1941), in a supporting role as one of the daughters of Benjamin Disraeli, played by John Gielgud.

The following year brought her most high-profile screen appearance, as Robert Donat's long-suffering second wife in the star-studded The Magic Box (1951), made as a project of the Festival of Britain.

Johnston was very complimentary about the René Clément-directed Knave of Hearts (1954), a Franco-British co-production of which she said: "The director was brilliant...it was a very sophisticated European film.

These were all one-off productions for BBC and ITV drama strands, featuring works by such as Shakespeare, George Bernard Shaw, Noël Coward and Jean Cocteau.

From 1965, Johnston was effectively running the business herself and decided, following her appearance in Sebastian, to give up her acting career in order to manage the agency full-time.

Following Parker's death in August 1974, Johnston continued to run the business for over 20 years, handling clients such as James Mason, Helen Mirren and Frank Finlay, until her own health started to fail.