The duchess - who was born in 1920 to Captain Evan Cadogan Eric Smith MC and his wife Beatrice Helen Williams - initially trained as a nurse at Great Ormond Street Hospital before marrying the then-Earl of Euston Hugh Fitzroy, who later became the 11th Duke of Grafton and died himself in 2011.
He went on to be succeeded by his grandson Henry Fitzroy, 43, who has served as the Duke of Grafton ever since.
The queen acknowledged the duchess's services by making her a Lady Grand Cross of the Royal Victorian Order in 1980 - an honour normally reserved strictly for members of the royal family.
The tragic news of her passing comes just months after the queen lost her husband Prince Philip, who died in April at the age of 99 following a marriage of 73 years.
The Duke of Edinburgh's passing left a "huge void" in the queen's life.
Their son, Prince Andrew, previously said: "The queen as you would expect is an incredibly stoic person. She described his passing as a miracle and she's contemplating, I think is the way that I would put it. She described it as having left a huge void in her life but we, the family, the ones that are close, are rallying round to make sure that we're there to support her."
