She never appeared as a stage actress, but had roles, mostly comic, in many films, including Miss Gossage in The Happiest Days of Your Life (1950) and Police Sergeant Ruby Gates in the St Trinian's series (from 1954).

Born to an affluent Anglo-American family, Grenfell had abandoned early hopes of becoming an actress when she was invited to perform a comic monologue in a West End revue in 1939.

[6] After this she enrolled at the Royal Academy of Dramatic Art in London, but found the hard work of learning the craft of acting less glamorous than she had imagined and left after a single term.

[14] At an informal supper given by the BBC producer Stephen Potter in January 1939, she agreed to his request to entertain her fellow guests with a monologue of her own devising.

The Stage judged her "outstanding ... this clever diseuse successfully catches the naif manner of an amateur speaker lecturing on 'useful and acceptable gifts', and gives us a neat and satirical impersonation of an American mother listening to her small daughter reciting Shelley's 'Ode to a Skylark'".

[18] The Bystander thought that Grenfell challenged the celebrated Ruth Draper "on her own pitch ... carry[ing] off the acting honours of this gay and intelligent entertainment.

Together they wrote many successful songs including "I'm Going to See You Today" and "Turn Back the Clock", which, in the words of the biographer Janie Hampton, "aptly caught the public mood".

[26] In late 1943 the head of ENSA, Basil Dean, invited the two to tour troop camps and hospitals in North Africa, the Middle East and elsewhere.

[20] Back in London Grenfell wrote the song "Du Maurier" (music by Addinsell) and the monologue "Travelling Broadens the Mind", both of which she performed in Noël Coward's first post-war revue, Sigh No More (1945).

"[31] In addition to her own two numbers, she sang Coward's comic catalogue of domestic disasters "That is the End of the News", "disguised as a schoolgirl with pigtails, all my make-up off, a shiny face and a terrible grin.

"[32] After the 1947 revue Tuppence Coloured, Grenfell developed new sketches including the first of her six Nursery School monologues, with the harassed teacher's recurring cry to one of her unseen charges, "George – don't do that...."[33] In the 1951 revue Penny Plain she performed her "Joyful Noise" (music by Donald Swann), a parody of an amateur choir ("And some of us cannot sing much, And some can't sing at all, But how we love our outings to the Royal Albert Hall").

[36] The Stage commented that any doubts that Grenfell could sustain a solo evening were quickly dispelled: After two provincial tours and a year in London she took the show to Broadway, where it had a sell-out eight-week run.

[37] During the 1950s and 1960s Grenfell appeared in several film roles including "Lovely Ducks", the shooting gallery attendant in Stage Fright (1950),[38] Miss Gossage in The Happiest Days of Your Life (1950), Police Sergeant Ruby Gates in the St Trinian's series, Mrs Barham in The Americanization of Emily and Hortense Astor in The Yellow Rolls-Royce.

Between 1957 and 1970 she gave her show Joyce Grenfell in Australia, Canada, Hong Kong, New Zealand, Singapore, Switzerland and the United States, as well as around Britain and in the West End.

After this Grenfell did not return to the stage, but gave talks for charitable organisations and appeared frequently on the BBC television programme Face the Music.