LONDON — Princess Elizabeth was never meant to become queen, let alone the longest-serving monarch in the storied history of British royalty.

Her father, in fact, was never meant to become king.

But on Monday, after nearly a lifetime of service to country and crown, Elizabeth II will commemorate her 65th anniversary as queen.

It is a role that most Britons — whether royalist or republican — would agree that she has fulfilled with caution, dignity and an unending sense of duty.

She will become the only British monarch ever to celebrate her Sapphire Jubilee.

44 PHOTOS Queen Elizabeth's style has changed See Gallery Queen Elizabeth's style has changed June 1927: A positively precious portrait of Princess Elizabeth, daughter of the Duke and Duchess of York. Photo: Hulton Archive/Getty Images January 1932: Princess Elizabeth, wearing a sharp little coat, walks in to visit the Royal Tournament at Olympia. Photo: Hulton Archive/Getty Images January 1933: Princess Elizabeth stands beside her younger sister, Princess Margaret Rose. Photo: Universal History Archive/Getty Images June 1936: Wearing a sweet pink dress, Princess Elizabeth looks quite charming, standing with her mother and sister. Photo: Hulton Archive/Getty Images January 1938: Princess Elizabeth standing with her grandmother, Queen Mary, matching her younger sister in a long trench coat and mary-jane shoes. Photo: Fox Photos/Getty Images June 1940: Princess Elizabeth looking studious (if unhappily so) at Windsor Castle. Photo: Lisa Sheridan, Getty Images May 1944: Princess Elizabeth wearing a summer dress and sunhat in the grounds of Windsor Castle. Photo: Lisa Sheridan, Getty Images February 1952: An official picture of when Princess Elizabeth acceded to the throne, becoming Queen, and wearing some impressive jewels. Photo: APIC/Getty Images December 1953: Talk about a long train! Her Majesty at her wedding to the Duke of Edinburgh. Photo: Keystone/Getty Images March 1954: Queen Elizabeth II on the balcony of Government House, Melbourne, during her tour of Australia. Photo: Fox Photos/Hulton Archive/Getty Images April 1954: The Queen in a floral-patterned dress for a tour of Aden, Yemen. Photo: Universal Picture Group/Getty Images November 1954: Queen Elizabeth dancing with Air Marshal Sir John Baldwin, at a ball held at the Hyde Park Hotel, London, 26th November 1954. The ball celebrated the centenary of the Battle of Balaclava. Photo: Dennis Oulds via Getty Images June 1956: Queen Elizabeth II in Sweden, wearing a lot of pearls and an interesting hat. Photo: M. McKeown, Hulton Archive/Getty Images January 1960: The Queen wearing a classic jacket as she stands with Prince Charles and Prince Edward at Windsor castle. Photo: Anwar Hussein, Getty Images July 1965: Queen Elizabeth II wore a vibrant coat and hat at Ventnor during a royal visit to the Isle of Wight. Photo: Fox Photos/Getty Images January 1969: The Queen, in a fuchsia dress, walks beside U.S. President Richard Nixon. Photo: Hulton Archive, Getty Images January 1973: Now that is glamour: from the intricate pattern, to the fur and crown, there's not much else we can ask for! Photo: Tim Graham, Getty Images February 1977: Queen Elizabeth looks like she's ready for spring in this plaid dress and bright hat. Photo: Serge Lemoine, Getty Images July 1982: Navy and white is always in style, and we appreciate the small pattern on her dress mixed with the bold lines of the hat. Photo: Tim Graham, Getty Images October 1986: Queen Elizabeth II stands out in robin's egg blue during a visit to Hong Kong. Photo: Tim Graham, Getty Images May 1993: The Queen and Prince Philip attend a formal banquet, the queen in a bright pink gown and a very expensive -- and we expect heavy -- accessory on her head. Photo: Tim Graham, Getty Images March 1995: The Queen visited South Africa and wore a bright yellow hat with this patterned dress. Photo: Tim Graham, Getty Images November 1998: London must be chilly in November! The Queen wore this large fur coat as she left the House of the Lords. Photo: Johnny Eggitt, Getty Images November 2002: The Queen and Prince Philip attend the premiere of the "Die Another Day" James Bond flick, dressed to impress! Photo: Nicolas Asfouria, AFP/Getty Images November 2003: U.S. President George W. Bush is formally welcomed to the U.K. by the Queen, dressed here in a lilac coat and hat. Photo: Kristy Wigglesworth, AFP/Getty Images June 2004: The Queen's hats keep getting better and better, and we love the top hat vibe this one has for Trooping the Colour. Photo: James Quinton, WireImage/Getty Images June 2005: A lady in pink always shows up men in black (though they look quite dapper as well!). Photo: Tim Graham, Getty Images October 2005: Her Majesty looked stunning in this white coat to visit the Tynemouth Volunteer Life Brigade Watch House and Museum. Photo: Anwar Hussein, WireImage/Getty Images May 2006: Queen Elizabeth II attends the Order of the Bath service, dressed in traditional garments, and looking lovely. Photo: Anwar Hussein, Getty Images June 2006: The Queen looks chic in this dress as she and Prince Philip attend a performance at the Royal Opera House. Photo: Tim Graham, Getty Images May 2007: The Queen dials up the fun of this look by wearing vibrant, neon pink in her accents, paired with a lavender wool coat. Photo: Chris Jackson, Getty Images October 2007: Her Majesty attends the State Banquet at Buckingham Palace, dressed in silk. Photo: John Stillwell, AFP/Getty Images June 2008: The Queen hosts a garden party at Buckingham Palace, and we're loving the sheer umbrella. Photo: Tim Graham, Getty Images October 2008: The Queen wore this intricately patterned dress during a two-day tour of Slovenia. Photo: Chris Jackson, Getty Images December 2008: The Queen attends the Christmas Day church service, looking lovely in a holiday-themed outfit. Photo: Chris Jackson, Getty Images June 2009: Her Majesty knows how to accessorize, and we can't get enough of her great looks! Photo: Indigo/Getty Images November 2010: The Queen wore this floral dress during a visit to Abu Dhabi. Photo: Julian Parker, UK Press/Getty Images April 2011: Prepping for the royal wedding in style, the Queen attends a dinner in this metallic and periwinkle gown. Photo: Danny Martindale, WireImage/Getty Images May 2011: Dashing leaders! Looking stunning in an embellished dress, Queen Elizabeth prepares to attend a State Banquet with U.S. President Barack Obama. Photo: Chris Jackson, AFP/Getty Images June 2012: Queen Elizabeth accessorizes this monochrome ensemble with vibrantly colored flowers. Photo: Julian Parker, UK Press/Getty Images June 2012: The Queen shares a laugh at the Queen's Cup Polo Day in 2012, dressed in the color of success. Photo: Nick Harvey, WireImage/Getty Images March 2013: The dark trim on this coat adds another level of sophistication, as if the Queen needed any help in that department! Photo: Mark Cuthbert, UK Press/Getty Images April 2014: This portrait of Queen Elizabeth by photographer David Bailey was released on April 20, 2014 to mark her majesty's 88th birthday on Monday April 21. Photo: Getty Images April 2015: The Queen attends a reception at Canada House on April 19, 2015 in London. Photo: Danny E. Martindale/Getty Images Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

In keeping with the habits of a lifetime, Elizabeth, who is 90, is expected to spend the day quietly at Sandringham, her country estate 110 miles north of London.

The course of Elizabeth's life was forever changed in 1936 when her uncle, Edward VII, abdicated to marry Wallis Simpson, a twice-divorced American commoner. His brother, Albert, became king, adopting for his reign the name George VI.

And Princess Elizabeth, who had spent the first 10 years of her life never expecting to become queen, suddenly found herself first in line to the throne.

With Edward's abdication, it fell to George VI — a shy man and a stutterer — to inspire his countrymen to endure the hardships of World War II, and to prevail. He was a smoker, and the strain took a toll on his health.

In February 1952, with her father ill, she and Prince Philip, her husband of five years, stood in for the king on an official visit to Kenya.

The couple took a brief and exhilarating rest at Treetops Hotel, nearly 6,500 feet above sea level, with a view of Mount Kenya.

On the night of February 5-6, the king died. Though she did not know it, Elizabeth was already queen — the first British ruler of modern times never to know the exact time of her assumption of office since the king died in his sleep.

"For the first time in the history of the world," her bodyguard, a hunter named Jim Corbett, wrote in the visitor's log book, "a young girl climbed into a tree one day a princess and after having what she described as her most thrilling experience she climbed down from the tree the next day a queen — God bless her."

Only, she did not know that until later in the day. At a hunting lodge, Phillip took the call. Her father had died, he told his wife, and she was already queen. She was 25 years old.

Upon returning to England, she met on Feb. 8 with the Lords of the Council for the formal proclamation of her reign as Her Majesty Queen Elizabeth II.

"By the sudden death of my father I am called to assume the duties and responsibilities of sovereignty," she said. "My heart is too full to say more to you today than I shall always work, as my father did throughout his reign, to advance the happiness and prosperity of my peoples."

In her early days as monarch, the queen relied on her husband and others to help her feel her way.

"The queen had a range of advisers right from the get-go," said Roya Nikkhah, the royal correspondent for the Sunday Times. "She had Winston Churchill as her first prime minister, an amazing relationship between the old prime minister at the end of his tenure and the new monarch at the beginning of hers."

But the young queen had views of her own, as well. Despite advice to the contrary, she decided that her coronation, on June 2, 1953, would televised.

It was an important move, Nikkhah said.

"Televising the coronation was groundbreaking for its time — to bring the monarchy into millions of peoples' homes against all of the advice of her advisers who said this makes the monarch look too day-to-day, too real," Nikkhah said. "She realized actually this is what she wanted to do, set the tone for her entire reign, making the monarchy relevant and bringing it to the people."

Further televised royal spectacles followed, such as the lavish 1981 wedding between Prince Charles — who has now be heir to the throne longer than anyone in British history — and Lady Diana Spencer.

And television is giving new life to those early days of Elizabeth's reign. The captivating young queen is portrayed in the successful Netflix series, "The Crown," which has introduced those exciting early years to a new generation.

"Throwing back to her as a very young queen, because now we are so familiar with her as a nonagenarian — I think that has probably drawn a lot of people to new interest in the monarchy, who are able to see what this woman has done, what she has achieved, from a very young woman through her 90s," Nikkhah said.

The queen, now perhaps the world's most prominent nonagenarian, had a very different persona 65 years ago, when she became queen, said Camilla Tominey, the royal editor for the Sunday Express.

"If you look at the queen now, you seen an elderly lady and we have done for decades now. (But) she was such a glamorous figure, along with her sister, Princess Margaret — these women who radiated beauty."

22 PHOTOS Queen Elizabeth and Duke of Edinburgh's wedding See Gallery Queen Elizabeth and Duke of Edinburgh's wedding Princess Elizabeth, later Queen Elizabeth II with her husband Phillip, Duke of Edinburgh, after their marriage, 1947. (Photo by Â© Hulton-Deutsch Collection/CORBIS/Corbis via Getty Images) UNITED KINGDOM - NOVEMBER 20: November 20, 1947: Princess ELIZABETH and the Duke of Edinburgh after the wedding ceremony. (Photo by Keystone-France/Gamma-Keystone via Getty Images) 20th November 1947: Princess Elizabeth and The Prince Philip, Duke of Edinburgh waving to a crowd from the balcony of Buckingham Palace, London shortly after their wedding at Westminster Abbey. (Photo by Keystone/Getty Images) Photograph of Princess Elizabeth and Prince Philip, Duke of Edinburgh on their wedding day. Dated 20th Century. (Photo by Universal History Archive/UIG via Getty Images) Sir Philip Mountbatten, newly titled Duke of Edinburgh (front, second from right), with fellow Royal Navy officers at his bachelor party at the Dorchester Hotel, London, 19th November 1947. Seated to the right of him is his uncle, Louis Mountbatten, 1st Earl Mountbatten of Burma (1900 - 1979) and to the left, Captain McGregor. Philip is to marry Princess Elizabeth (later Queen Elizabeth II) the following day. (Photo by Keystone/Hulton Archive/Getty Images) 1947: H R H Princess Elizabeth and Philip Mountbatten, Duke of Edinburgh, on the occasion of their engagement at Buckingham Palace in London. (Photo by Fox Photos/Getty Images) 1947: Princess Elizabeth and the Duke of Edinburgh. (Photo by Central Press/Getty Images) Newly married Princess Elizabeth and Prince Phillip, Duke of Edinburgh during their honeymoon in Malta. (Photo by Â© Hulton-Deutsch Collection/CORBIS/Corbis via Getty Images) July 1947: Princess Elizabeth in the state apartments at Buckingham Palace during her engagement to The Prince Philip, Duke of Edinburgh. (Photo by Fox Photos/Getty Images) Princess Elizabeth (later Queen Elizabeth II) and naval Lieutenant Philip Mountbatten (later Duke of Edinburgh) photographed for the first time since the announcement of their engagement, 10th July 1947. (Photo by Central Press/Hulton Archive/Getty Images) 14th November 1947: Mr Schur, chief confectioner at McVitie and Price, putting the final touches to the wedding cake of Princess Elizabeth and The Prince Philip, Duke of Edinburgh. The cake has four tiers and is nine feet high. (Photo by J. A. Hampton/Topical Press Agency/Getty Images) UNITED KINGDOM - NOVEMBER 20: Wedding Of Princess Elizabeth And The Duke Of Edinburgh. Princess Elizabeth And The Duke Of Edinburgh On The Balcony Of Buckingham Palace. To The Right Of The Princess Is The Marquess Of Milford Haven, Witness The Duke Of Edinburgh On November 20Th, 1947 (Photo by Keystone-France/Gamma-Keystone via Getty Images) UNITED KINGDOM - NOVEMBER 20: Wedding Of Princess Elizabeth And Duke Of Edinburgh. Photo Taken At Buckingham Palace. On November 20Th, 1947 (Photo by Keystone-France/Gamma-Keystone via Getty Images) UNITED KINGDOM - NOVEMBER 20: Wedding Of Princess Elizabeth And Duke Of Edinburgh. The Royal Procession Arriving At Buckingham Palace. On November 20Th, 1947 (Photo by Keystone-France/Gamma-Keystone via Getty Images) 20th November 1947: Princess Elizabeth, and The Prince Philip, Duke of Edinburgh at Buckingham Palace after their wedding. (Photo by Hulton Archive/Getty Images) UNITED KINGDOM - NOVEMBER 20: Wedding Of Princess Elizabeth And Duke Of Edinburgh. On November 20Th, 1947 (Photo by Keystone-France/Gamma-Keystone via Getty Images) 20th November 1947: Princess Elizabeth leaving Westminster Abbey, after her wedding to The Prince Philip, Duke of Edinburgh. (Photo by Topical Press Agency/Getty Images) 29th July 1949: Princess Elizabeth and her husband Philip, Duke of Edinburgh complete their tour of West Riding in Yorkshire with a visit to the city of York. (Photo by PNA Rota/Getty Images) 21st October 1950: Princess Elizabeth (later Queen Elizabeth II) holding her daughter Princess Anne at her christening in Buckingham Palace, with the Duke of Edinburgh at her side. (Photo by Topical Press Agency/Getty Images) Prince Philip, the Duke of Edinburgh, and Princess Elizabeth following the Archbishop of Canterbury in a procession to Canterbury Cathedral, Kent, July 31st 1949. (Photo by Fred Morley/Fox Photos/Getty Images) CANADA - JANUARY 01: Canada, Princess Elizabeth And The Duke Of Edinburgh Posing With Canadian Mounties In 1951 (Photo by Keystone-France/Gamma-Keystone via Getty Images) Undated picture showing the Royal British couple, Queen Elizabeth II, and her husband Philip, Duke of Edinburgh, with their two children, Charles, Prince of Wales (L) and Princess Anne (R), circa 1951. (Photo credit should read OFF/AFP/Getty Images) Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

Decades of duty have defined the queen. She has traveled more than a million miles, visited about 120 countries, and met with 12 U.S. presidents, from Eisenhower to Obama. And, at the age of 90, she continues to perform her royal duties, carrying out 80 public engagements in 2016.

She has served with dignity. A major part of her job, not to put too fine a point on it, has been to go 65 years without publicly saying anything inappropriate. It is a task at which her husband, Prince Philip, who is 95, has occasionally failed, with his penchant for ethnic jokes that are awkward at best.

One bobble, though, was the queen's failure to react quickly to Diana's death in a car crash in 1997. Diana and Charles were divorced, the queen was not thought to hold Diana in high regard, and the flag at Buckingham Palace was not flown at half-staff.

However, the queen recovered her footing and made a speech expressing admiration for Diana and her grandmotherly concern for Princes William and Harry.

After so many years on the throne — longer than most Britons have been alive — many people have come to respect her quiet devotion to duty, and her determination to keep her private life out of sight.

Even after 65 years on the throne, during a televised reign in which everyone came to know her love of horses and corgis, people know little about the queen as a person.

"With the queen, you have this dichotomy — that there was this very public figure, the most photographed woman in the world, combined with somebody who behind palace gates we didn't know much about at all as a woman," Nikkhah said. "She has always kept her cards held close to her chest."

The country is celebrating this anniversary by issuing a new five-pound coin. It will bear the words the queen uttered nearly 70 years ago, the day she turned 21: "My whole life, whether it be long or short, shall be devoted to your service."