Lady Henderson, who has died aged 84, had a full working diplomatic partnership with her husband, Sir Nicholas Henderson, throughout his ambassadorial posts in Warsaw, Bonn, Paris and Washington.
In Warsaw, she quickly learned Polish to be able to talk to Edward Gierek, Secretary of the United Workers Party, with whom she went to football matches. In Washington she was able to reveal much more than Sir Nicholas could find out for himself about what was going on in Ronald Reagan's mind by regularly sitting next to the President at dinner.
But Mary Henderson was also very much a figure of significance in her own right. An obvious public instance of this was the creative drive she was to put into promoting and reinvigorating a British fashion and design industry which in the late 1970s and early 1980s had been somewhat in decline.
She took great care to refurbish, redecorate and, where necessary, partly reorganise the interior of each embassy to her own high standards of taste, and she did so without any expense to the British public. Selecting British designs and materials from suppliers she admired, she found them ready to provide these free in return for the international status and acclaim the work brought them.
When the embassy days were over, Mary Henderson became an active member of the so-called "Gang of Six" (which also included Caroline Charles, Terence Conran, Jean Muir, Beatrice Miller and Roy Strong) which eventually persuaded Mrs Thatcher to support fashion and design enterprise with government finance, setting up, for example, the Silver Trust. Her part in such achievements was not always widely recognised, although she was appointed OBE.
Much of the strength of her personality derived from the fact that, although both her parents were Greek, she was brought up by a loved English governess, and was happy to become in spirit simultaneously both Greek and English, acquiring in youth the nickname "Xenia" - Greek for "foreign" - which was later used as the title of her memoir, published in Britain in 1988.
She was born Mary Cawadias in Athens on March 29 1919. The Greek-English trend of her early development was much strengthened when in the mid-1920s, a time of political disturbance in Athens, her father, Professor A P Cawadias, a distinguished pediatrician in charge of a large teaching hospital in the Greek capital, decided to move with his family to London; there he retook a medical degree and set up an important practice in Wimpole Street. His daughter found herself able, like her governess, to think of this as "coming home".
Mary went to Queen's College in London, and passed her entrance exam to study Medicine at Oxford, to which her father did not want her to go, because he did not want "a blue-stocking daughter". He was not to have one. Instead of going up to Oxford, she went with her mother on holiday to the country of her birth, with which she at once fell deeply in love, and where circumstances were to determine that she would stay for the next all-important years of her life. For while they were there, Britain went to war with Germany, and her father wrote to tell them to stay in Greece, to avoid the dangers of German heavy bombing.
Before long, however, war hit Greece itself. Mussolini's Italian attack through Albania was followed by a German one, and later by a bitter civil war with Elas Communist guerrillas. Mary served a good part of this time as a Red Cross nurse in Athens. But with their known English connection, Mary and her mother inevitably became suspect to the Germans. They were arrested, and put in a makeshift German concentration camp near Athens, where hostages and other prisoners were regularly shot, and where execution could be expected by anyone. Taken to an interrogation centre, she was subjected to a brief but painful experience of torture and returned to the main camp to wait for whatever might happen next.
This was a complete surprise. Sent for again after a time, she was told by a German officer that she and her mother were to be released. They quickly learned why: British troops had landed and, under the command of George Jellicoe (thereafter a good friend), were successfully approaching Athens.
She soon got herself a job as a war correspondent for Time-Life, and was to continue in it throughout the ensuing Greek civil war with the former Elas Communist guerrillas, learning her trade in the company of the foreign correspondent Patrick O'Donovan, of the Observer. She also met a British foreign official posted to Athens named Nicholas Henderson. They were married in London on December 19 1951.
Before departing with him on his foreign appointment to Madrid, she continued to work as a journalist on a new magazine, Africa. Mary Henderson had a quiet stamina and in-built perfectionism, and was kind and helpful to friends and acquaintances alike. She was a wonderful cook, who bewildered with her talent the excellent chef she had in Paris - between them they turned the kitchen into a cooking school. In 1980 she published Paris Embassy Cook Book, typically describing herself as an amateur.
Mary Henderson died on January 22. She is survived by her husband and by their daughter, Alexandra, Countess of Drogheda, whose husband is the photographer Derry Moore.
Patrick Leigh Fermor writes: To the first allied harbingers of the liberation, Mary Henderson, and her friend Marina Ladas, seemed to personify the Greek spirit. The two girls seemed to set the dark years to rout, and prove that civilisation, beauty, poetry, all the arts and all the charm and the fun were in place again. Mary's wide and expressive eyes, and her smile and her quietness, seemed to fill life with promise, and many hearts were won. Her vigour disguised as frailty came to her help in the darker days of the civil war.
Then she wrote for Time-Life, a path which led to adventurous and dangerous slogs in the Pindus mountains by Jeep, on foot, or on horseback. She was a gifted writer, as Xenia, her autobiography, amply proves.
The temple at Sunium, at the tip of Attica, is an auspicious place to meet. So it proved with the advent of Nico Henderson. Their marriage was a merging of backgrounds: Wimpole Street, Queen's, Kolonaki and Byzantium on one side, and a stoic post-Stracheyan Bloomsbury, Anglo-Ireland, academe, the FO, bohemia and the Gargoyle on the other. And it carried them dashingly and informally through a succession of exalted appointments.
A few months ago she corresponded with me by setting down the text in English, transliterated for fun into Greek characters; or, alternatively, set down in Greek, but using English characters, both with hilarious results. She loved this kind of thing, and creeping deafness failed to hamper a verve which her friends will bitterly miss.