She was born Frances Ruth Roche in Park House, on the royal estate at Sandringham, Norfolk, on 20 January 1936. Her father was Maurice Roche, 4th Baron Fermoy, a friend of King George VI and the elder son of the American heiress Frances Ellen Work and her first husband, the 3rd Baron Fermoy. Her mother, Ruth Roche, Baroness Fermoy, was a confidante and lady-in-waiting to Queen Elizabeth (later the Queen Mother). Since birth, she held the style of The Honourable as the daughter of a baron.
On 1 June 1954, she married John Spencer, Viscount Althorp (later the 8th Earl Spencer), at Westminster Abbey. The Queen and other members of the royal family attended the wedding ceremony. She was 18 years old and became the youngest woman wed in Westminster Abbey in the last five decades.
Her marriage to Viscount Althorp was not a happy one and, in 1967, she left him to be with Peter Shand Kydd, an heir to a wallpaper fortune in Australia whom she had met the year before. His half-brother was the former champion amateur jockey william Shand Kydd (1937-2014), who was the brother-in-law of John Bingham, 7th Earl of Lucan. Subsequently, she was named "the other woman" in Janet Shand Kydd's divorce action against her husband.
Frances and Peter Shand Kydd were married on 2 May 1969 and lived on the Scottish island of Seil, where they bought an 18th-century farmhouse called Ardencaple, just 10 kilometres from Oban. She divided her time between London, Seil and another sheep farm in Yass, New South Wales. Although she lived a quiet life, she was forced into public view following the engagement of her daughter Diana to Prince Charles on 24 February 1981.
On 14 July 1976, the 8th Earl Spencer married Raine, Countess of Dartmouth, daughter of the Novelist Dame Barbara Cartland. He eventually won a bitter custody battle over his children. Shand Kydd and her second husband separated in June 1988, after he left her for a younger woman. She blamed the pressure of media attention for the breakdown of the marriage.
In 1996, she was banned from driving after being convicted of drunk-driving, but denied she had a Problem with alcohol. She and Diana quarrelled in May 1997, after she told Hello! magazine that Diana was happy to lose her style of "Royal Highness" following her controversial divorce from Prince Charles. She was reportedly not on speaking terms with her daughter by the time of Diana's death.