Born in Llantillio Pertholey in Gwent, Cecily came from a coal-owner family with military connections. Her paternal great-grandfather, Sir Digby Mackworth, one of Wellington's officers, married Julie de Richepense, daughter of one of Napoleon's generals. Her maternal grandmother was born and brought up in the Victorian English colony in Dieppe. Cecily was four when her father, an army officer, was killed in action early in the first world war. Her mother remarried and moved to Sidmouth.
Cecily managed to get through several governesses. When she was 18 her aunt Margaret, Viscountess Rhondda, editor of the weekly Time and Tide and a governor of the LSE, found a place there for Cecily. She stayed for two years. Her first love was a Hungarian LSE student, Nicky Kaldor (the economist Lord Kaldor), who became a lifelong friend.
Aged 22, Cecily married Leon Donckier de Donceel, a young Belgium lawyer she met at a Swiss sanatorium, where both were being treated for tubercolosis. Widowed at 25, she was left with a daughter to bring up, a role for which she was entirely unsuited. She returned briefly to England. Her first poems were published in the London Mercury, but she found England dull and wanted to travel.
The loveless nature of her family was illustrated when Cecily's sister Helen and her fiancé and his mother were found dead in Donegal from shotgun wounds in mysterious circumstances. Cecily recalled her mother being asked: "What shall we do with her dog?" Her mother blithely replied: "Shoot it as well". Helen was buried in a distant part of a cemetery in Ireland, which none of the family ever went to visit. The dog was apparently buried with her.
Cecily left England, and spent much of the early 1930s in Hungary and Germany, before moving to Paris in 1936. She was in Berlin for the burning of the Reichstag, an account she wrote up but could not get published.
In the summer of 1937 she was taken to meet Henry Miller, then living at the Villa Seurat, in his ground floor studio in the Rue de la Tombe Issoire. He took to her and she became part of his Paris circle. Through Miller she met 25-year-old Lawrence Durrell, newly arrived from Corfu with the manuscript of his novel, The Black Book.
"Send me everything in your jam cupboard," Durrell once wrote on a scrap of paper he slipped under her door. She showed him her poems, which Miller published - Eleven Poems (1938) - as an offshoot of his magazine the Booster, of which three numbers had appeared. The fourth, subtitled Air-conditioned Womb Number, brought it to an abrupt end.
A fraught love affair with an exiled Czechoslovak painter led to two books on his country, which she suppressed from her bibliography. She stayed in France until June 1940 when the German victory necessitated her escape via Spain and Portugal.
Her first big success was in 1941, with a vivid account of that defeat, I Came Out of France. TS Eliot read it and invited her to tea in Russell Square, interested to meet the young woman. She worked for a spell with the Free French in Carlton Gardens in 1940, in the office of Colonel Howard, the head of security.
She was at Carlton Gardens throughout the blitz, and she remembered the restless atmosphere of ill-temper in the building. There was distrust between various factions of the French and British intelligence services. It was there she met Colonel Passy, whose real name was Dewavrin, chief of De Gaulle's secret service. She also became close to the future French foreign minister Maurice Schumann, who was at that time head of the press office. He broadcast twice a day to France, Les Français parlent Aux Français, which included coded messages to Free French agents.
Cecily remained in London for most of the war, where she gave lectures to the army. She contributed poems and articles to the literary magazine Horizon and other reviews.
She moved back to France after the war and published François Villon: a Study (1947) and compiled A Mirror for French poetry 1840-1940: French Poems with English Translation by English Poets (1947) . She also worked as a journalist, visiting many countries as a correspondent. In 1947 she went to Palestine for the journal, L'Aube. She returned in 1948, travelled throughout the Middle East, and wrote an account of this wartorn region in The Mouth of the Sword (1948).
In 1952 she published her first novel, Spring's Green Shadow. Her second, Lucy's Nose, appeared 40 years later. Of her own works she was most fond of The Destiny of Isabelle Eberhardt (1954). In spring 1950 she had gone to Algeria to follow in Eberhardt's footsteps and travelled alone in the Sahara. Her research led to the first biography of the young woman, who was born in 1877 and brought up by Trophimovsky, a half-crazy former Pope in the Orthodox Church. Eberhardt became a Moslem, lived the life of an Arab nomad, disguised as a man, and was initiated in an esoteric sect of Islam. Eberhardt died, aged 27, in a desert flash flood.
Cecily married the Marquis de Chabannes La Palice in 1956. She had lived in Paris since the 1940s. During the next 30 years she published widely books and reviews. Notably, Guillaume Apollinaire and the Cubist life (1961) received the Darmstadt Award. English Interludes, (Paul Verlaine, Stéphane Mallarmé, Paul Valéry, Valery Larbaud - stays in London 1860-1912) was much praised. Her contribution to Mallarmé studies is substantial.
Of her earlier memoir, Ends of the World (1987), Lawrence Durrell wrote to Cecily: "Just reading, sipping, devouring to prolong the pain/pleasure of your beautiful, cogent and brilliant memories."
She published on various occasions reminiscences of her friendships, of writers and of the literary scene from the 1930s to 1950s: at 93, she embarked upon her candid autobiography. It was typical of her, that in her last weeks, with her autobiography just completed, she was in search of a new project, this time to embark on learning Arabic.
She is survived by her daughter, Pascale.
Cecily Joan Mackworth, writer, born August 15 1911; died July 22 2006