For Meghan's joint engagement with the Queen on June 14, she wore an ivory Givenchy dress with a high neckline with caped shoulder. For the private ceremony at St. James's Palace, the Duchess of Sussex wore a monochrome olive green look by Ralph Lauren.
See both of Markle's looks below.
The newly minted Duke and Duchess of Sussex (AKA Harry and Megan) followed behind the new family of five, along with Louis' proud grandpa, Prince Charles, who walked in with his wife Camilla, Duchess of Cornwall.
Check out some of the photos of the stunning Meghan Markle and her fellow royals at Westminster.
Kate, 36, on the other hand, opted to wear an Alexander McQueen coat and a hat by Sean Barrett.
The look was a departure from the toned-down colours worn by Meghan after her wedding.
The event marked the 100th anniversary of the world's oldest Air Force - the Royal Air Force - which was formed on April 1, 1918, independent of the British Army and Royal Navy.
The pair are being formally welcomed to the Emerald Isle by Ireland's Taoiseach Leo Varadkar and are due to enjoy a summer garden party at Glencairn, the official residence of Britain's Ambassador to Ireland Robin Barnett, where the duke will give a speech.