For William, the ceremony served as an early 26th birthday gift (his actual birthday is June 21). But for his girlfriend of about six years, it was another first to be included in so grand an occasion.
While Middleton had attended William's graduation from army academy Sandhurst and his ceremonial finish of Royal Air Force training, she has not been at an event of such royal importance led by the Queen.
William's new title – he was made the 1,000th member of the ancient Order of the Garter – signifies his joining the oldest order of chivalry in the world.
Middleton's arrival in a royal car with William's brother, Prince Harry, and the princes' personal secretary, Helen Asprey, was accompanied by the theme from Pirates of the Caribbean played by a military band.
Army officer Harry "was acting like he was on guard duty," standing between Middleton and the cameras, an observer says. The trio then stood in the shadows of Galilee porch on the side of St. George's chapel, where the service took place.
Also in the throng was the princes' stepmother Camilla, the Duchess of Cornwall, and other female members of the family.
The Order of the GarterWilliam received the Order of the Garter in private from the Queen on May 5 and then Monday – supported by his father, Prince Charles, and grandfather, the Duke of Edinburgh – he was invested in a private ceremony in Windsor Castle's Throne Room.
Army officer William is currently on a two-month re-assignment to the Royal Navy, which will see him heading to the Caribbean to take part in anti-drug running patrols.
The awarding of the Order underlines once more how William is moving towards becoming a senior royal, who will eventually take on more duties on behalf of his grandmother once he leaves the armed forces.