The Duchess of Cornwall, who entered the service with her husband Prince of Wales, wore a white A-line dress by Fiona Clare and a cream wide-brimmed Locke hat.
"It's clearly a style that she feels comfortable with, which is so important when you're stepping out in front of the cameras."They were joined at the ceremony by the Queen and other senior members of the Royal Family, including Prince Charles and Prince William.
Check out some of the photos of the stunning Meghan Markle and her fellow royals at Westminster. The air force's actual 100th birthday was in April, but today's celebrations arrive 100 days after the anniversary, according to the RAF's website.
The Duchess also pushed boundaries by going bare-legged at the official photocall to announce her engagement to Prince Harry.
The festivities in London include a service at Westminster Abbey followed by a parade of up to 1,300 serving personnel and a celebratory flypast with up to 100 aircraft over Buckingham Palace.
Nearby nine Garda motorbike outriders were waiting to escort the couple to a meeting with Ireland's Taoiseach Leo Varadkar, who will formally welcome the couple to his country. Meghan's signature face-framing tendrils were nowhere to be seen.