Other members of the family including the Prince of Wales, Duchess of Cornwall, Duke of Cambridge, Duke of York, and Duke and Duchess of Sussex attended a reception at Buckingham Palace where they mingled with RAF veterans, serving personnel and their families.
Earlier, the royals and more than 2,200 people packed Westminster Abbey for a service also attended by Theresa May, defence secretary Gavin Williamson and Labour leader Jeremy Corbyn.
Look familiar? It should, as it's the same Alexander McQueen design she wore three times before in a creamy white shade for Prince Harry's and Meghan Markle's wedding in May, Trooping the Colour in 2017 and Princess Charlotte's christening in 2015.
After the service at the Abbey, the royals went to Buckingham Palace to watch a flypast of aircraft to mark the centenary celebrations. Markle paired the Audrey Hepburn-esque number with a vibrant Stephen Jones fascinator and beige pumps.
The Duchess of Sussex joined senior royals at Westminster Abbey in a custom Dior, a day after being panned for wearing "drab" green for Prince Louis' christening.
The flypast was followed by a solemn rendition of the national anthem, "God Save the Queen", as members of the military formed themselves into the phrase "RAF 100" on the terrace below the palace balcony.
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The Queen looked radiant in a royal blue silk coat with a turquoise collar over a dress in the same colour, both by Angela Kelly, with a matching hat and a diamond star.
Missing was the queen's husband, Prince Philip, 97, who has retired from public life.
The Royal Flying Corps and the Royal Naval Air Service merged to create the RAF, the world's first independent air service, on 1 April 1918.
She later remembered the Battle of Britain, saying: "I remember the Battle of Britain being battled in the sky above us and we should never forget the sacrifice".
The largest formation of RAF Typhoons, a combat aircraft, spelled out 100 above Buckingham Palace.