Seven years on from her own nuptials, Middleton stepped out in what the palace described as a "primrose yellow wool silk tailored coat" by Alexander McQueen (who also designed her wedding dress).
The Duchess of Cambridge wore the same stunning coat dress to Charlotte’s christening and the Queen’s birthday.
“The Duchess of Cambridge is wearing a hat designed by Philip Treacy and shoes by Jimmy Choo," Kensington Palace said in a statement.
Middleton walked hand-in-hand with the royal family's youngest members, who served as flower girls and pageboys.
Pageboys John and Brian Mulroney were excited to accompanied Markle in her car.
Meanwhile, Prince William played best man to Harry; they both wore the frockcoat uniform of the Blues and Royal.
Kensington Palace confirmed back in April that Wills would serve as best man to his little brother, just as Harry did for him at his wedding in 2011.
"Prince Harry has asked his brother The Duke of Cambridge to be his Best Man at his wedding to Ms. Meghan Markle," Kensington Palace confirmed via Twitter.
Though the announcement came just weeks out from the royal nuptials, Harry revealed that William has known about the important role "for months," joking, "I got down on one knee" to pop the question.
During a visit to the Greenhouse Center charity in London, reporters asked William how he felt to be standing beside his brother on his wedding day, to which the father-of-three replied: “It feels great. [I'm] thrilled and delighted, honestly.”
Wills couldn't help but throw a joke in, adding, “Revenge is sweet”.