The off-shoulder style has become part of her signature look, with the Duchess opting for elegant, simple outfits since becoming a member of the royal family.
She started her morning attending a service for the RAF at Westminster Abbey alongside her husband, The Queen and other senior royals, before the royals returned to Buckingham Palace.
Although Kate, 36, isn't due to return to work until September, she chose to make an appearance at the event after RAF made her an Honorary Air Commandant in 2015, passing the role from Prince Phillip to her, according to the Mirror.
More than 2,200 people packed Westminster Abbey for the service, with dignitaries including Prime Minister Theresa May, Defence Secretary Gavin Williamson and Labour leader Jeremy Corbyn attending.
A video posted by the Kensington Palace showed Kate Middleton commenting on how Prince Louis was "very relaxed and peaceful" to the Archbishop of Canterbury. She also wore the diamond and ruby RAF Dacre brooch, which was given to her in 2015.
Kate last wore the broach - once the highest award any woman of the rank could achieve - during a service for the 75th anniversary of the RAF Air Cadets in 2016. She wore a gorgeous pastel blue dress by Alexander McQueen with her RAF brooch pinned to her lapel.
The royals will then appear on the balcony of the Queen's London residence to view a flypast of up to 100 RAF aircraft, a parade of more than 1,000 personnel, and a feu de joie.
Those recognised as the founders of the RAF include Lieutenant General Sir David Henderson and Lord Hugh Trenchard - the first chief of the air staff.
It has also been noted that Kate's decision to wear white or cream at the christening of Prince George, Princess Charlotte, and now Prince Louis is due to the colours perfectly complementing the Royal christening gown, worn by all three children.
In her speech, the Queen said, "I remember the Battle of Britain being fought in the skies above us, and we shall never forget the courage and sacrifice of that time".
It was another huge moment for Meghan, who has been adapting to life as a member of the royal family since she married Harry at Windsor Castle on May 19.