The correspondence dates from when Mary was Countess of Harewood, having married Henry Lascelles, the sixth Earl of Harewood, in the 1920s and moved to his ancestral home on the outskirts of Leeds.
Mary was the only daughter of King George V and two of her five brothers would later become king. She was also the aunt of our present Queen.
The contents of the letters to and from her siblings and friends, which were found in a basement at the stately home, form the basis of Channel Four documentary series The Queen's Lost Family. The first of three episodes aired on Sunday.
Goldsborough Hall: The B&B near Knaresborough that was Princess Mary's first family homeIt is the first time the Princess Mary Archive, which is on a long-term loan to the Harewood House Trust, has been explored in detail for television, although Mary's possessions have been included in past exhibitions at Harewood.
The letters cover controversial topics such as the abdication of her eldest brother, King Edward VIII, in order to marry his divorced American girlfriend Wallis Simpson, and the passing of the crown to their younger sibling Albert, who took the throne as King George VI. Edward's letters make reference to his tour of the British Empire in 1921 and his dissatisfaction at the increasing public duties in preparation for his future as king.
The papers also touch on affairs enjoyed by the younger sons of King George V, Prince Henry and Prince George. Henry had a liaison with aviator and racehorse trainer Beryl Markham, who lived in colonial Kenya, which his parents disapproved of, and George, who died in a plane crash during World War Two, was rumoured to have had bisexual relationships in the 1920s, including one with novelist Barbara Cartland. Edward was also notorious for his womanising as a young man. Both Henry and George went on to marry and some of their children, who are cousins of the Queen, still perform public engagements on her behalf.
The secret history of Bradford's underground riversAlbert - known as Bertie, who had married Elizabeth Bowes-Lyon, later the Queen Mother, and had two daughters, Princess Elizabeth and Princess Margaret, and Mary, who had two sons with Henry Lascelles and lived a quieter life on his Yorkshire estates - are portrayed as the more staid and settled of the four adult siblings. Their youngest brother, Prince John, suffered from epilepsy and died in childhood.
The letters cover three decades of social history, from the 1920s until the end of World War Two. The archive consists of 170 boxes of items, including letters written by Mary when she was just five years old.
Press cuttings and diary entries are also included in the cache, which has already been studied in detail by staff at Harewood over the past 20 years, although it has yet to be fully catalogued.
The Harewood Houst Trust plans to mark the TV series with a new exhibition about Princess Mary's life which will begin in September and will explore her family relationships as a daughter, sister, wife and mother.
Assistant curator and archivist Rebecca Burton said:-
"Highlights include the intimate correspondence between Princess Mary and family members following her engagement in 1922, and a display of the newly-conserved train of Princess Mary’s wedding dress, embroidered with emblematic flowers of the British Empire, alongside her bridal slippers and a floral headdress.
"The exhibition will appear throughout several rooms on the State Floor and Below Stairs, and follows heightened increase in Princess Mary, and the release of the Downton Abbey film, which was filmed at Harewood House."
The Downton Abbey feature film, which is due to be released in cinemas in September, was inspired by the royal visit of Princess Mary's parents to Wentworth Woodhouse, near Rotherham, in 1912, when she was a child, although the movie is set in the late 1920s. It was shot on location at both Wentworth and Harewood, and Harewood's terrace appears in the trailer.
The Queen's Lost Family is on Channel Four on Sunday nights at 8pm.
Is Hayburn Wyke the most beautiful picnic spot in Yorkshire? Princess Mary at Harewood
Princess Mary married Henry Lascelles in 1922, and as he was still heir to his father's title, their first marital home was at Goldsborough Hall, near Knaresborough. They raised their sons George and Gerald there, and King George V and Queen Mary were regular visitors. George Lascelles was the monarch's first grandchild.
By 1930, Henry had inherited the earldom and they moved to Harewood House. He died in 1947 and Princess Mary continued to live at Harewood with her son George and his family. She acted as chancellor of the University of Leeds and performed some royal duties. She suffered a heart attack while walking in the grounds of Harewood in 1965, and died at the age of 67. Her funeral was held at York Minster and she is buried on the estate. Her grandson David is the current Earl of Harewood.
King George V's grandchildren - including George Lascelles, seventh Earl of Harewood, who died in 2011 - are all first cousins of Queen Elizabeth II.
The children of King George V
The lives of King George V and Queen Mary's children captivated the British public during the inter-war period. Five of their grandchildren are still alive today.
Edward, the eldest, was a 'playboy prince' who enjoyed affairs with married women. He ascended to the throne in 1936, but was king for less than a year before abdicating due to the constitutional crisis that was precipitated by his intention to marry Wallis Simpson, who had two living ex-husbands. He was granted the title Duke of Windsor and although he performed official duties during the war, he later moved to France and rarely returned to Britain. He died in Paris in 1972, with no children.
Albert, the second son, had never expected to become king, but was crowned at the end of 1936. By this time, he was already married to Elizabeth Bowes-Lyon and had two daughters, Elizabeth and Margaret. He adopted the title George VI - George was one of his middle names. His stammer is the subject of the film The King's Speech. He died in 1952 and his throne passed to Queen Elizabeth II.
Mary was the third child and only daughter. She spent most of her adult life in Yorkshire after her marriage to Henry Lascelles, and they had two sons, George and Gerald. She died in 1965. Gerald died in 1998 and George in 2011.
Henry, the third son, was the first British royal to attend school rather than be educated at home. After his affair with Beryl Markham, which caused a public scandal, he married Lady Alice Montagu Douglas Scott and they had two sons, William and Richard. Henry's title was Duke of Gloucester. After the war he served as governor-general of Australia. He was the last of the six siblings to die, in 1974. His eldest son William predeceased him by two years, having been killed in an air crash, so the dukedom passed to Richard.
George, the fourth son, was known for his active social life and friendships with celebrities. Like Edward, he also enjoyed a playboy lifestyle, but by 1934 had settled down and married Princess Marina of Greece. He held several military and diplomatic roles and used the title Duke of Kent. He had three children - Edward, Alexandra and Michael, all of whom are still living. In 1942 he was killed along with 13 others when the RAF Short Sunderland flying boat he was aboard crashed in Scotland. He was 39. Edward inherited the title of Duke of Kent.
John was the 'lost prince' of the family, whose life was the subject of a 2003 television drama written by Stephen Poliakoff. He was the youngest child, born in 1905, and suffered from epileptic seizures. He was kept away from the public eye and spent most of his time on the Sandringham estate. He died in 1919, at the age of just 13.