It was entirely characteristic of her and of the tone of many of her relationships that when in 1980 Philip Toynbee, one of her oldest friends, wrote to inform her that he had been asked to prepare her obituary her response to his request for information was crisp. "Goodness you've a nerve," she wrote. "In other words I should write the obit and you collect the fee whilst retaining what Film Folk call creative control over the content?" It was even more characteristic that when she sent him a list of her books she invented one in order, as she put it, "to test your ability as researcher/obit-writer".
Her love of such tricks sprang, like her distaste for sentimentality, from her childhood. She was never much interested in nature or the countryside, but she was born in 1917 in one of the loveliest landscapes in England at Asthall Manor, a handsome Jacobean house near Burford on the edge of the Cotswolds. Jessica Lucy Mitford, always know to friends and family as Decca or Dec, was the penultimate child and fifth of the six daughters and one son - all strikingly good-looking and strong-willed - born to the second Lord Redesdale and his wife Sydney. Her father built a larger and uglier house nearby at Swinbrook, where Jessica - along with Nancy, Pam, Tom, Diana, Unity and Deborah - played her own provocative observant part in the teases and rituals that have, as a result of her own and her sisters' writings, anecdotes and behaviour over 50 years, become a minor legend.
Her eldest sister Nancy wrote the first lightly fictionalised account of the Mitford family in The Pursuit of Love, an instant success when it was published in 1945; Decca enhanced the legend with her first book, Hons and Rebels, in 1960. Her account of life at Swinbrook was full of jokes but painted a less sunny picture than did Nancy's; and they could never agree on the derivation of the famous Hons' Cupboard, where the children used to gather and plot. Nancy's version is that as they were all Honourables, being the children of a peer, they knew each other as the Hons; Decca, not being keen on class honorifics, states firmly that Hon was another word for Hen, as they were devoted at the time to a family of chickens.
During the 1930s the nursery games and the politics of the schoolroom became serious, as Jessica found herself repelled by her sister Unity's addiction to Nazi ideas and increasingly drawn herself to the socialist cause. Whether they really did compete in decorating their rooms with posters and portraits of Hitler and Stalin hardly matters; what is incontrovertible is that by the time she was 18 Jessica had found the basis for the political and social beliefs that were to shape her life and was planning to run away to put them into practice.
In 1937 she made headlines by eloping to Spain with Esmond Romilly, a nephew of Winston Churchill's and another adolescent rebel from the ranks of the upper classes, who had joined the International Brigade to fight against Franco. Neither her family's pursuit nor high-level diplomatic intervention had any effect and they were married by the British Consul in Bayonne. When they returned to England they went to live in the East End of London, not then a fashionable area, where they joined the Bermondsey Branch of the Labour Party. They had a daughter, Julia, who caught measles and died at the age of four months.
By the time the Second World War began in 1939 a huge gulf separated Decca's views and way of life from the rest of her family. Her sister Unity was a passionate admirer of Adolf Hitler and a frequent visitor to Nazi Germany, as was her sister Diana, who had married the British Fascist leader Oswald Mosley in 1936. Her brother Tom - who was to be killed in 1945 in Burma - was also a Fascist sympathiser, as were both her parents.
Like many left-wingers, the young Romillys at first saw the looming war between England and Germany as irrelevant to the revolutionary struggle and early in 1939 they left for America. The outbreak of war found them confused by the Nazi-Soviet pact, but Germany's attack on Russia in 1941, Decca later wrote, "changed everything overnight". Leaving her and their new baby daughter Constancia, nicknamed Dinky, with friends in Virginia, Esmond joined the Canadian Air Force and left for England. He was killed in action in November 1941.
After Esmond's death Decca moved to Washington where she found a job with the Office of Price Administration and met the man who became her second husband in 1943, a Harvard-educated lawyer of Hungarian-Jewish extraction, Robert Treuhaft. They shared political opinions, a delight in subversive activities of all kinds and liked the same jokes; they made a perfect, if at first unlikely, team. After the war they settled in Oakland, close to San Francisco and the Berkeley campus, where Treuhaft's legal practice specialised in labour and racial cases that establishment law firms preferred to avoid. They had two sons: Nicholas, born in 1944, and Benjamin, born in 1947. Again Decca had to endure the death of a child in 1955 when Nicholas was killed in a traffic accident. She preferred never to speak of it.
The Treuhafts had both joined the Communist Party during the war, and Decca soon began to do her bit at meetings and study groups. She became particularly active in the growing Civil Rights movement, to which her commitment was total. Her account of her political activities is given in her second autobiographical book, A Fine Old Conflict (1977); comic, irreverent and only slightly disingenuous, it shows how her adherence to the party line was always moderated by her natural individualism and anarchic streak as well as her instinctive distaste for jargon. In 1956, in the wake of Nancy Mitford's invention of U and Non-U language, Decca produced a pamphlet entitled Lifeitselfmanship, or how to become a precisely-because man: an investigation into current L (or Left-wing Usage).
But for all her capacity to mock the comrades, Decca worked hard for the party until she and Bob decided to leave it in 1958, and she showed great moral courage in standing up for her beliefs both during the McCarthy period and during the ugly Civil Rights battles of the 1950s and 1960s. She would admit later that the Party had made mistakes, but nothing would ever make her apologise for it. "I can hardly imagine," she wrote, "living in America in those days and not being a Party member." Recently, it appeared that the collapse of the Soviet Union caused her neither rejoicing nor regret.
After the success of Hons and Rebels, Decca, with Bob Treuhaft's crucial help and support, turned herself into an accomplished and best-selling professional writer. Her second and perhaps her best book was the macabre and hilarious expose of the funeral racket, The American Way of Death (1963), which established her as a brilliantly effective critic of her adopted society. Thereafter she wrote The Trial of Dr Spock (1969), Kind and Usual Punishment (1974), The American Prison Business (1975), Poison Penmanship: the gentle art of muckraking (1979), Faces of Philip: a memoir of Philip Toynbee (1984), Grace Had an English Heart: the story of Grace Darling (1988) and The American Way of Birth (1992). She became a hugely popular lecturer to American student audiences, who were fascinated by her unique blend of imperious Mitford charm, fierce radicalism and mannered English voice; she was also increasingly in demand for television programmes, where her strong performing streak came into its own.
From the late 1950s she was able to return on frequent visits to England, usually built around and subsidised by a project. No one was better suited or more delightfully ruthless than Decca at finding a freebie, a lift, some research help; but if she could be a demanding friend she was always a generous and stimulating one. She renewed affectionate if sparring contact with all her surviving sisters (Unity shot herself in 1939 and died in 1947) except Diana Mosley; the implacable streak in Decca's nature revealed itself when in answer to an offer from Lady Mosley to entertain her teenage son Benjy on a trip to France she responded that she thought not: she did not want him turned into a lampshade.
On regular visits to London she would gather her Old Left cronies, mix them with younger friends and relations most, but not all, of various shades of pink, and regale them with lethal Martinis and Bob's special Boston baked beans before visiting her sister Deborah, the Duchess of Devonshire, at Chatsworth for the weekend. She was a constant letter-writer and faxer and craved news and gossip. "I long to know all - do tell" was a frequent cry.
Decca enjoyed nothing more than to launch into a song or two after dinner, usually a medley of left-wing ballads and 1930s favourites. Not long ago her great friend Maya Angelou flew into London on Concorde for the day to attend a Decca party where the two of them put on a memorable performance of "Right Said Fred".
Last year her friends received a tape, with a childhood snap of a scowling Decca on the front, from Don't Quit Your Day Job Records of San Francisco, containing Decca and the Dectones' version of "Maxwell's Silver Hammer" and "Grace Darling" (trad.).
Decca Mitford made few concessions to age or weakness. Refusing to admit that her uncharacteristic frailty and various aches and pains were serious, she was at work until shortly before she died on her latest project, a new edition of The American Way of Death.
Jessica Lucy Mitford, writer and campaigner: born Burford, Oxfordshire 11 September 1917; married 1937 Esmond Romilly (died 1941; one daughter, and one daughter deceased), 1943 Robert Treuhaft (one son, and one son deceased); died Oakland, California 23 July 1996.