Deborah, the Dowager Duchess of Devonshire, and the last surviving Mitford sister, has died aged 94.
Her son, the Duke of Devonshire, announced the death in a statement from Chatsworth House, her stately home.
Prince Charles said he was "saddened" by the news, saying he "adored and admired" Deborah and would "miss her so very much".
The Mitford sisters fascinated - and sometimes scandalised - British society in the 1940s.
Unity was a friend of Hitler; Diana, the second wife of British fascist leader Sir Oswald Mosley; Jessica, a left-wing activist, and Nancy, a novelist and historian.
Deborah, like her elder sister Pamela, was more focused on home life.
Nonetheless, along with her siblings, during her lifetime she moved in the same circles as Sir Winston Churchill, John F Kennedy and Evelyn Waugh.
She also accompanied her sister Unity to tea with Hitler in 1937, was painted by Lucian Freud, and amassed a collection of Elvis Presley memorabilia.
Nicknamed the "housewife duchess", she made Chatsworth in Derbyshire one of the most successful and profitable stately homes in England after marrying Andrew Cavendish - who later became the 11th Duke of Devonshire - in 1941.
The statement from her son said: "It is with great sadness that I have to inform you that Deborah, Dowager Duchess of Devonshire, has passed away peacefully this morning."
It added that an announcement about funeral arrangements would be made shortly.
Prince Charles said: "My wife and I were deeply saddened to learn of the death of the Dowager Duchess of Devonshire, whom both of us adored and admired greatly.
"She was a unique personality with a wonderfully original approach to life, and a memorable turn of phrase to match that originality.
"The joy, pleasure and amusement she gave to so many, particularly through her books, as well as the contribution she made to Derbyshire throughout her time at Chatsworth, will not easily be forgotten and we shall miss her so very much."
Born Deborah Vivien Freeman-Mitford on 31 March 1920, the duchess was the sixth daughter of the 2nd Baron Redesdale.
The Mitfords' childhood at their family home in the Oxfordshire village of Swinbrook was immortalised in Nancy's novels, The Pursuit of Love and Love in a Cold Climate.
Her parents made a poor job of hiding their disappointment that Deborah had not been born a boy, leaving Thomas their only son.
The Mitfords' father disapproved of educating girls, famously insisting that hockey would make their ankles fat, and Deborah spent her formative years skating and hunting.
Her sister Unity's infatuation with Hitler saw the young Deborah invited to tea with the German dictator, although the visit made little impression on her.
"If you sat in a room with Churchill," she later recalled, "you were aware of this tremendous charisma. Kennedy had it too. But Hitler didn't - not to me anyway."
At Chatsworth, the Duchess took on a major role in running the house and its garden, which have been used in a number of film and TV productions, and she ran the estate's farm shop.
In July 2002, the duchess and her husband spoke out against the government's proposed ban on fox hunting.
Made a dame in 1999, she became the Dowager Duchess of Devonshire in 2004 after her husband died and their son inherited his title.
She penned a number of books including the autobiographical Wait for Me: Memoirs of the Youngest Mitford Sister, which was published in 2010.