As a boy in Canada, Ivan Watson was captivated by the stories that his grandfather told him about his ancestors back in Britain.

The family had inherited a large cache of faded photographs, documents and heirlooms, each painting a vivid picture of life across the Atlantic. But there was always one picture that stood out.

“I was about 10 years old when my grandfather first opened a suitcase of old photographs and took out a signed portrait of Queen Mary, wife of George V,” Ivan tells Who Do You Think You Are? Magazine.

“He then started to tell me about his aunt – also named Mary – who had apparently worked for the queen as her private nurse.”

So how did an ordinary young woman from Scotland become a royal confidante?

After years of vowing to find out more, Ivan finally started investigating his great great aunt’s life two years ago. Thanks to a “perfect blend of oral history and archival research”, he has now managed to establish the facts – and the story is even more impressive than he anticipated.

My grandfather opened a suitcase of old photographs and took out a signed portrait of Queen Mary

The second of five children, Mary Sangster McDonald was born in 1868 and grew up in the busy port city of Aberdeen, where her father served in the merchant navy.

Her first husband, Peter Watson, passed away in 1898, but she found happiness again when she married John McBain – one of Peter’s first cousins.

In 1901, having spent several years employed as a steamship nurse stewardess, Mary was selected to serve aboard the royal yacht HMS Ophir on its world tour.

“According to family legend, Mary was standing in a formal reception line of nurses when the future queen (then Duchess of Cornwall and York) came for an inspection,” says Ivan. “She picked Mary out, and invited her to come work for her as her private nurse.

“It was an extraordinary and unheard-of honour for a woman such as Mary, and she soon accompanied the duke and duchess on their overseas duties.”

The highlight of Mary’s career came in 1911, when she was asked to join the newly crowned king and queen on a three-month voyage to India aboard the RMS Medina.

The centrepiece of the tour was the Delhi Durbar, where the royal couple were proclaimed emperor and empress of India in a ceremony dubbed “the greatest spectacle in the history of the British Empire”.

Upon their return, each member of the travelling entourage was presented with a gift in recognition of their service. But Mary, who had now sailed with the queen on three occasions, was singled out for special praise.

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“Mary was the only female recipient of the king’s Delhi Durbar Medal, out of 30,000 awarded,” explains Ivan. “In fact, when the voyage ended at Portsmouth in 1912, the king pinned the medal to her dress personally, and Queen Mary gave her a gold watch.”

While they may have come from very different walks of life, the women had a strong connection. Mary’s knowledge of nursing, combined with her innate sense of hospitality, were a great source of comfort to the queen, who sometimes suffered from seasickness.

After many years of service, Mary retired from her duties and settled down with her husband in Grimsby. However, the queen continued to send her a signed photograph every Christmas – including the crinkled portrait that so fascinated Ivan.

Mary and John were also invited to attend the coronation of George VI in 1937. Ivan’s cousins, Brenda and Hazel Watson, still vividly remember “Auntie Mary and Uncle Mac” recounting the story of the big day.

Mary passed away in 1952, aged 83. Knowing the close bond that had existed between his late wife and her former employer, John wrote to Queen Mary to let her know. In her reply the monarch expressed her condolences, along with her appreciation for Mary’s service.

Ivan is proud that his relative was so highly regarded.

“Mary enjoyed a particularly remarkable career for a woman of her era and background, and her story deserves to be remembered. It proves that anything is possible in life with a little bit of luck, ability and old-fashioned hard work. That’s why she is my family hero.”

Jon Bauckham is a freelance journalist based in Bristol, UK. He holds a degree in history and was previously the features editor at Who Do You Think You Are? Magazine and a section editor at BBC History Magazine. You can follow him on Twitter.