Since Kate is on maternity leave, she wasn't expected to attend the event, though Westminster Abbey confirmed she would be there when they released the order of service on their Twitter account in the early hours of Tuesday morning.
The Duchess of Sussex looked stunning in a demure, belted black dress from Dior with her favored boatneck neckline and a wild, Stephen Jones fascinator that she makes look chic.
Later, Harry and Meghan, along with Prince William and Duchess Kate of Cambridge, appeared at a reception at the palace. On Monday, Prince Louis was christened at the Chapel Royal in St James's Palace surrounded by his family.
Kate wore an icy blue Alexander McQueen coat dress and a matching hat from Sean Barrett.
And as such the pretty piece of jewellery was given to the Duchess as a gift on her appointment.
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Kate joined her husband and other members of the Royal Family for a service at Westminster Abbey followed by a presentation of new Queen's Colour to the Royal Air Force and a flypast viewed from the balcony at Buckingham Palace.
The duchess wore one of her tried and true favorite brands, Alexander McQueen, marking the third time in a row that she has worn McQueen to her children's christenings.
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.
The RAF, described as the world's first independent air force, was founded on April 1, 1918 as an armed force independent of the British Army and the Royal Navy.