Deborah Mitford, the Dowager Duchess of Devonshire, was the last of six famous sisters in an upper-crust British family whose antics left her countrymen shocked, appalled and always entertained. She died Sept. 24 at 94.

The death was announced by her son, Peregrine, who did not release the location or cause. She lived near the Derbyshire estate of Chatsworth House, the neoclassical palace that she and her husband, the 11th Duke of Devonshire, rescued from its post-World War II nadir and revived as one of England’s finest country houses.

Her role in Chatsworth’s fortunes alone marked her for posterity, but she will be remembered as the sixth and youngest of the Mitford sisters, whose outlandish behavior and political radicalism seemed to make them characters contrived by one of their chums, the satirical novelist Evelyn Waugh.

Two of her sisters, Diana and Unity, were friends of Adolf Hitler; their eldest sibling, Nancy, was a best-selling novelist, and another, Jessica, was a Communist who moved to California and wrote her own bestseller, “The American Way of Death” (1963), a wry look at the underhanded practices of the funeral industry. Pamela led the quieter life of an upper-class countrywoman, devoted to her dogs and her garden.

In a real-life, upper-class soap opera that made TV series “Downton Abbey” look positively uneventful, Deborah Mitford — often called “Debo” — emerged as a long-suffering peacemaker between her squabbling siblings.

Deborah Mitford, seen here in 1941, was a peacemaker between her squabbling siblings, whose real-life, upper-class soap operas made the TV series “Downton Abbey” look uneventful. (AP)

In her late-life memoir, “Wait for Me!” (2010), she sought to explain if not condone the enduring pre-World War II friendship between Hitler and Diana, a political admirer of the Nazi dictator, and Unity, the one groupie who could make the Führer giggle.

“I do not share [Diana’s fascist] views, but my love for her overcame this side of her character,” she wrote. “Unity was always the odd one out.”

Selina Hastings, a biographer and authority on the Mitford family, called Deborah Mitford “by far the most balanced of all the sisters. She was down to earth.”

Early life, eccentric father

The sisters’ saga proved a great trial to their aristocratic parents, David and Sydney Redesdale. “I’m normal, my wife is normal, but my daughters are each more foolish than the other,” Redesdale told friends. This account may not have been shared by others. “Farve,” as he was known, set a model of eccentricity for his children.

He once banished a house guest for displaying a hair comb and had a morbid fear of anything gooey. When Deborah asked her father what his idea of hell was, he replied, “Honey on my bowler hat.”

Deborah Vivian Freeman Mitford was born March 31, 1920, in the village of Asthall in the English Cotswolds and grew up in an age of dynamic social change and upheaval following World War I.

Her father’s fluctuating fortunes caused a series of moves into ever-smaller homes, but Deborah Mitford nevertheless enjoyed a childhood that was essentially of her class and privileged status.

She spent Saturdays fox hunting and Sundays ice skating. She was schooled by a governess and pursued a childhood of social grooming that would lead to the prize — a season of debutante balls, presentation to the queen and the quest for a husband, preferably titled.

The conformity was shaped, too, by early manifestation of each sisters’s peculiarities and distinctly different personalities. Nancy was engrossing but cruel — she called Deborah “Nine” because she felt her intellect had stopped developing at that age. The closest to Waugh, Nancy Mitford went on to write two blockbuster satires in the 1940s that drew heavily on her family — “The Pursuit of Love” and “Love in a Cold Climate.”

Diana Mitford married the heir to the Guinness brewing fortune. Jessica — Decca — was Deborah’s “boon companion” in their formative years, perhaps because they were polar opposites. Decca was a bookworm who eschewed nature and beauty. Unity was strange and recalcitrant.

Deborah’s cosseted if quirky early life began to unravel when she was 12. With two young children, Diana left her husband for Sir Oswald Mosley, a debonair politician who abandoned conventional politics in the 1930s to found and lead the British Union of Fascists.

Diana and Unity first met Hitler in 1933, the year he rose to power in Germany. Unity fell in love with him, Diana saw him as an ally to Mosley’s cause and even their mother found Hitler alluring and charming.

Recognizing that Decca was resentful and unhappy at home and in full embrace of Communism, Sydney Redesdale arranged a world cruise for her and Deborah. But the treat for Deborah was ruined when Decca eloped beforehand with Esmond Romilly, the 18-year-old nephew of future prime minister Winston Churchill and a veteran of the Spanish Civil War. The quest to find them, aided by the British navy, was salaciously covered by the popular press in London.

War and personal trauma

When Britain declared war on Germany in 1939, Unity followed through with her threat to shoot herself, symbolically in the Englischer Garten in Munich. (She was living in Munich in a flat Hitler’s lieutenants made available by evicting a Jewish couple). The bullet lodged in her brain, but she survived. Hitler sent her to a clinic in Switzerland and, a few weeks later, Deborah and her mother traveled to bring her home.

Deborah’s angst mounted — Diana and Mosley, by then married, were imprisoned by the British government as enemies of the state, Unity eventually died from her injuries, and Lord and Lady Redesdales were riven by their opposing views of Hitler. She supported him.

“For Debo, the only child left at home, this was a traumatic time,” Mary S. Lovell wrote in “The Sisters” (2001), a biography of the Mitford siblings. “She was the only witness of the effect on her parents of Decca’s elopement, the worry over Unity, and the continuous disagreement between them about Hitler.”

Shortly after turning 21, Deborah found an exit and married Andrew Cavendish, a young Guards officer and younger son of the Duke of Devonshire. Amid the privations and dangers of wartime London, their wedding cake had a white cardboard collar to give the appearance of icing. The reception was in a ballroom where the windows had been blown out days before by a German bomb.

In 1944, her husband’s brother, Billy, was killed in action in Belgium, meaning that Andrew and Deborah would become the 11th Duke and Duchess. Billy was married to Kathleen “Kick” Kennedy, sister of the future president, a connection that placed Deborah in the Kennedy circle; they would later be guests of President John F. Kennedy in Washington.

Deborah’s only brother, Tom Mitford, died fighting the Japanese in Burma. Romilly died in Europe on a bombing mission.

Chatsworth, country life

Andrew and Deborah inherited the vast Chatsworth estate in 1950. This held in its own frightening prospects, but it gave Deborah, then 30, a life after the family saga of her earlier years and the personal costs of the war. She immersed herself in the country life of dogs, horses, shooting and livestock, of agricultural shows and equestrian events, and in a way she fulfilled a destiny as the chatelaine of a great country house.

The great challenge facing her and her husband was how to resuscitate the houses and estates that came with the dukedom, principally Chatsworth, a honey-stoned palace with 297 rooms, more than an acre under roof and 112 fireplaces.

It had been neglected and used and abused as an academy during the war. More to the point, the postwar tax laws in Britain imposed a punitive levy of 80 percent of the value of the estate.

They resolved to save Chatsworth by selling a second 16th-century palace, Hardwick Hall, along with many works of art. It took 24 years to settle the debt.

As the matron of one of the most important historic homes in England, she found that she had an entrepreneurial streak that turned the once-moribund property into a thriving enterprise, with 600,000 visitors a year.

She started a farm shop, renovated hotels and restaurants for visitors, and helped to initiate an agricultural show and equestrian events. The house’s setting in the idyllic Peak District of Derbyshire made it a favorite for film crews drawn to its backdrop, at a price.

Deborah wrote openly of her husband’s descent into alcoholism and described how she finally confronted it in the 1980s and issued an ultimatum: the booze or me. “For two decades, until the day he died,” she wrote in her memoir, “he never had another drink.”

Besides her son, survivors include two daughters, Lady Emma Tennant and Lady Sophia Topley; eight grandchildren; and 18 great-grandchildren.

After being widowed in 2004, Deborah moved back to the old rectory near Chatsworth where they had lived in the 1950s. It was open once a year to visitors, who could look in the loo and find a framed portrait of one of her heroes, Elvis Presley.