Frances Shand Kydd, who has died aged 68, had several incarnations. First, as a teenage bride; then, as a bolter, who fled an unhappy marriage, only to lose her second husband to a younger woman; then, as the mother of Princess Diana; and, finally, as a Roman Catholic convert who devoted her life to charity work among her co-religionists in Argyll, a close-knit community which treated her with respect and helped her cope with her inner demons. Friends say this last phase gave her the most satisfaction, and a degree of peace.
She was born into privilege and royal connections. Her father was the fourth Baron Fermoy, a shooting friend of George VI. Her mother, Ruth, was a woman of the bedchamber and confidante of Queen Elizabeth, the Queen Mother. In adult life, Frances's relationship with her mother was fragile; it became an open rift in 1969, when Lady Fermoy testified against her daughter in the acrimonious divorce from Viscount Althorp, later the 8th Earl Spencer. A bitter battle followed, in which Frances lost the custody of her four children.
She had been just 18 when, in 1954, she married "Johnnie" Althorp, 30 years old, heir to the earldom of Spencer, a captain in the Royal Scots Greys, equerry to the Queen, and acting master of the royal household. Home was Park House, on the Sandringham estate.
Frances gave birth to two daughters, in 1955 and 1957; a son, John, was born in 1960, but lived for only 10 hours. The pressure to produce a male heir was ever present, but 18 months later Diana was born. Subsequently, Frances, still only 23, was persuaded by the family to consult a gynaecologist to see why she produced only healthy girls. The much wanted son, Charles, the present Earl Spencer, was born in 1964.
He was three when his parents' marriage broke down, and, decades later, claimed that the gynaecological tests were probably the root cause. He did not think, he said, that his parents "ever got over it". Eventually, Frances left Althorp to marry wall-paper tycoon Peter Shand Kydd. During the divorce hearing, she was described as a marriage breaker and an adulteress.
The Shand Kydds eventually settled on the Scottish Isle of Seil, close to Oban, where Frances opened a gift shop, appearing content to live a life of relative obscurity. She integrated well into Scottish ways, although her 19-year-old marriage ended when Shand Kydd left her for a younger woman.
Diana's marriage to the Prince of Wales in 1981 punctured her privacy, and she became a target in the press frenzy swamping her youngest daughter. She blamed the failure of her second marriage on the unsought limelight, saying: "I think the pressure of it all was overwhelming and, finally, impossible for Peter. They didn't want him. They wanted me. I became Diana's mum, and not his wife."
Her son, meanwhile, said that "Diana's marriage moved us from the shadows of the landed aristocracy into the role of bit-part players in the soap-opera fantasy world that the media foisted on to the royal family."
Following her second divorce, and as Diana's marriage unravelled, Frances converted to Catholicism in 1994. She immersed herself in good works, and made annual pilgrimages to Lourdes, as a carer to groups of disabled people. She organised fundraising for the bereaved families of the Lockerbie air crash, and, as patron of the Mallaig and North-West Fishermen's Association, subscribed to Scottish Fishing Monthly to keep up with the issues.
She resigned from the Conservative party when Britain abstained on a Brussels vote that allowed Spanish trawlers into local waters, and publicly protested against cutsbacks in the coastguard service.
She drove long distances to comfort the families of fishermen aboard the sunken trawlers, the Sapphire and the Silvery Sea. As a mother who had lost two children, she went out of her way to support other mothers whose children had died tragically.
When Mohamed Al Fayed accused her of being "a bad mother, a snob, and behaving like the Queen of Sheba," after they had ignored each other at the Paris inquiry into Diana's death, Frances refused to respond. Instead, she sought out the parents of the chauffeur Henri Paul and, embracing them, said they had suffered "the sorest ache of all". She, herself, had been banned from driving for a year for drink-driving in 1996, though she always denied having a problem with alcohol.
She was devastated by Diana's death, a grief compounded by an estrangement caused by a magazine interview she had given, four months earlier, dislosing personal details about her daughter. The interview raised £30,000 towards the building of a Catholic house of prayer on Iona, but it made no difference, and Diana was said to have returned several of her mother's letters unopened.
Frances was haunted by the fact that there was no time for a reconciliation before the Paris crash, but was pleased to find she had remained an executor of Diana's will, and been assigned special responsibility for overseeing the upbringing of Princes William and Harry.
Her daughters, Lady Sarah McCorquodale and Lady Fellowes, and her son, the 9th Earl Spencer, survive her.
· Frances Ruth Burke Roche Shand Kydd, born January 20 1936; died June 3 2004