As the 24 Knights of the Order of the Garter made their way through the grounds of Windsor Castle wearing full regalia, among their numbers were Prince William, Prince Charles, the Duke of York and the Earl of Wessex.
Queen Elizabeth II and the Duke of Edinburgh followed them into St George's Chapel.
Duchess of Cambridge and the Countess of Wessex
The Duchess of Cornwall, Camilla, was unable to attend due to sinusitis, Clarence House confirmed.
Kate continued her tradition of recycling her favourite outfits, wearing a custom-made blue Christopher Kane coat which she wore to the Opening Ceremony of the 2012 London Summer Olympics, and nude-coloured heels.
The elegant Duchess also donned a Lock & Co hat in the same hue as her coat, while the Countess wore a short-sleeved white dress with a fur stole and pale pink hat.
Hundreds of spectators watched as the Garter Knights processed through the castle grounds to the chapel.
The oldest and most senior British Order of Chivalry, the Order of the Garter honours those who have held public office, have contributed to national life, or have served the monarchy personally.
After many years of the Order being chosen based on recommendation from government, it has now returned to its original function, as a sign of being favoured by royalty.
There can only be 24 knights at any one time and on Monday two new members of the Order were given the honours: Mervyn King, the former governor of the Bank of England, and Eliza Manningham-Buller, ex-head of MI5.