Patricia Mountbatten married celebrated film producer Lord Brabourne in 1946, and the pair had seven children.
The couple were among a seven-strong boat party blown up by the IRA at County Sligo, Republic of Ireland, in 1979.
Their son Nicholas, 14, was killed in the attack, as was Lord Brabourne's mother, Dowager Lady Brabourne, and her father Lord Mountbatten.
The Countess, then known as Lady Brabourne, was badly injured but her husband survived as did Nicholas's twin, Timothy.
Paul Maxwell, a 15-year-old local boy, was also killed.
For more than 30 years she used her experience of her loss to help other bereaved parents, through her support of the charities Child Bereavement UK and Compassionate Friends.
A spokeswoman for the family said the countess died peacefully on Tuesday at her home in Mersham, Kent, surrounded by her children.
Lord Brabourne passed away 12 years ago.
She was godmother to the Prince of Wales who paid tribute to her in a statement. It reads: "I was deeply saddened to learn of the death of my very special godmother, Lady Mountbatten, whom I have known and loved ever since I can first remember.
"She played an extremely important part in my life and I shall miss her presence most dreadfully."
A Buckingham Palace spokeswoman said: "The Queen and the Duke are aware and have privately passed on their condolences."
When the Countess married in 1946 the then Princess Elizabeth, her third cousin, was one of her bridesmaids.
The family lived at Broadlands in Romsey, Hampshire, near Southampton, which has a hospice named after Countess Mountbatten.
The arrangements for a funeral in London followed by a burial service in Mersham, are to be announced at a later date.
Prince Philip's cousin, the Countess Mountbatten of Burma, has died aged 93.