Mary, the fourth daughter and eleventh of fifteen children of King George III and Charlotte of Mecklenburg-Strelitz, was born on April 25, 1776 at the Queen’s House (now Buckingham Palace) in London. She spent most of her time in her mother’s household with her sisters as Queen Charlotte was loath to lose any of her daughters to marriage. Of the six sisters, only three married. Charlotte, Princess Royal married at age 29 which was a rather late age for marriage, but Mary was 40 and her older sister was 48 at the time of their marriages.
On July 22,1816, Princess Mary married her first cousin, Prince William Frederick, Duke of Gloucester and Edinburgh, the son of King George III’s brother, Prince William Henry, Duke of Gloucester and Edinburgh. Their marriage was childless but was extremely happy and William Frederick died after 18 years of marriage. The couple lived at Bagshot Park, now the home of Queen Elizabeth II’s youngest child Prince Edward, Earl of Wessex. Mary was a great favorite with all the members of the royal family particularly with her niece Queen Victoria. She was renowned for her charm and kindness. Mary died at age 81, the longest lived and the last survivor of her parents’ 15 children in 1857. She also has the distinction of being the only child of King George III to be photographed. Princess Mary was buried in the Gloucester Vault at St. George’s Chapel, Windsor Castle.
Wikipedia: Princess Mary, Duchess of Gloucester
The Gentleman’s Magazine and Historical Chronicle for the Year 1857: Obituary of HRH The Duchess of Gloucester