LONDON, July 1--Nancy Mitford, the prolific essayist, novelist and historian whose writing was enlivened by satire and a firm British aristocratic perspective, died yesterday at her home in Versailles, France. She was 68 years old. Unabashedly snobbish and devastatingly witty, Miss Mitford achieved enormous success and popularity as one of Britain's most piercing observers of social manners. Beginning with fiction that V. S. Pritchett once hailed as helping to begin "an aristocratic revival in English literatures," Miss Mitford moved on to finely observed histories, particularly of court life in France and Russia, and to widely enjoyed essays and translations. But through all her writing, she never let her readers lapse into unawareness of her own aristocratic, sheltered upbringing--the object of much of Miss Mitford's scalding satire but a background, nonetheless, which she took very seriously and continued to defend. In one of her most recent books, "The Sun King," which is a portrait of Louis XIV's life at Versailles, Miss Mitford unhesitatingly compared the plumbing at Versailles with what she had known on her own visits to Buckingham Palace in 1923. Indeed, one of Miss Mitford's pet concerns entered the history of obscure literary debates when, in 1955, she published perhaps her most famous essay on upper-class and non-upper- class forms of speech. The essay sparked such a controversy in Britain, with responses from many major literary figures, that Miss Mitford was compelled a year later to bring out a thin book, "Noblesse Oblige," with her disquisition on the subject as its centerpiece. Her argument, a set-piece even today among literary parlor games, was that the more elegant euphemism used for any word is usually the non-upperclass thing to say--or, in Miss Mitford's words, simply non-U. Thus: It is very non-U to say "dentures"; "false teeth" will do. Ill is non-U; sick is U. The non-U person resides at his home. The U person lives in his house. And so forth. Perhaps Miss Mitford and only a few others would have had the credentials to engage in this kind of argument. She was the oldest of six daughters of Bertram Ogilvy Freeman-Mitford, the second Baron Redesdale, who lived with Lady Redesdale at Swinbrook, the family estate in Oxfordshire. The girls called their father "Old Subhuman." "My father and mother, illiterate themselves, were against education, and we girls had none though we were taught to ride and to speak French," Miss Mitford wrote in "Twentieth Century Authors." "I grew up as ignorant as an owl, came out in London and went to a great many balls." "Here I met various people who were not ignorant at all--I made friends with the sort of people which included Messrs. Evelyn Waugh, John Betjeman, Sir Maurice Bowra and the brilliant Lord Berners. Very soon I became an intellectual snob. I tried to educate myself, read enormously and wrote a few indifferent novels." Miss Mitford was not the only family member to win fame. In America, her most well- known sibling is her younger sister, Jessica, the author, who wrote of the girls' childhood in her own memoirs, "Daughters and Rebels." Miss Mitford's first novel, "Highland Fling," in 1931, was--like many that followed--a "comedy of manners" based on her own experiences. It was followed by "Christmas Pudding," "Wigs on the Green" and "Pigeon Pie"--all exhibiting what some critics felt was a style more akin to "schoolgirl burble" than acutely tuned observation. Better received were "Pursuit of Love," 1945; "Love in a Cold Climate," 1949, and "The Blessing," 1951. These were sometimes frankly sentimental but possessed of a wit that Phyllis McGinley, the poet, found "quite funny and rather frightening." Among the victims of her humor were Americans of any kind. Eventually Miss Mitford moved to history--"by way of fiction," as Louis Auchincloss put it. In 1954 she wrote a biography of Madame de Pompadour and in 1966 her study of Louis XIV. Her most recent book, "Frederick the Great," was published three years ago. "She seems to have brought a new talent to the study of history," Mr. Auchincloss wrote in 1969, "that of the sophisticated, worldly wise observer, who is able to penetrate old archives with a fresh eye for qualities in the dead that she is specially qualified to recognize." "In general," another observer wrote, "the women. . .are judged more leniently than the men." Throughout her life, Miss Mitford did little traveling beyond the boundaries of her own country and France. Although one of her sisters, Unity, who died in 1948, became an enthusiastic admirer of Adolf Hitler, Nancy Mitford hated dictatorships and worked for the Republicans in the Spanish civil war. After World War II, she decided to settle in France. Miss Mitford was married to Peter Rodd in 1933 but the marriage ended in divorce in 1958. Her only brother, Tom, was killed during the war. She is survived by four of her five sisters, Pamela, Diana, Jessica and Deborah. She was a frequent visitor to London in recent years, visiting with friends who were among Britain's leading literary figures. "Today," The Times of London said yesterday, "they find their world colder and less merry: like Beatrice in 'Much Ado,' she was born under a star that danced."