WATCH William and Kate in action at the prestigious event:
The Duchess of Cambridge joined senior members of the British royal family, as well as visiting royalty King Willem-Alexander and Queen Maxima of the Netherlands and King Felipe and Queen Letizia of Spain, for the celebration of the oldest and most senior order of knighthood in the UK, People Magazine reports.
Kate, 37, wore the ivory 'Liza' dress coat by one of her favourite designers Catherine Walker, which was delicately embellished with a black scallop trim.
The mother-of-three - who left little ones Prince George, 5, Princess Charlotte, 4 and one-year-old Prince Louis at home the for the day - paired the number with a striking black fascinator from Lock & Co. and a simple black clutch.
The Duchess wore her brunette tresses in a chic chignon and accessorised with an elegant pair of pearl drop earrings.
The Duchess of Cambridge was seen in deep conversation with Queen Letizia and her step mother-in-law, Camilla, Duchess of Cornwall outside the Chapel.
Later, Kate chatted with Queen Maxima.
The Duke of Cambridge was joined by his father Prince Charles for the symbolic parade, with both men donning the iconic midnight velvet robes and ornate ostrich plume hats which are the signature of the order.
The historic event, which includes a service at St George's Chapel, is hosted each year by Her Majesty Queen Elizabeth II.
Also in attendance were Prince Andrew, Prince Edward and his wife Sophie, Countess of Wessex, as well as Princess Anne and the Queen's cousin Princess Alexandra.
Two notable absences from yesterday's festivities were the Duke and Duchess of Sussex.
Meghan Markle is still on maternity leave with newborn baby Archie Harrison, but Town and Country reports she almost definitely would not have attended regardless because Prince Harry has not yet been made a Knight of the Order of the Garter.
Royal expert Marlene Koenig told Harper's Bazaar it's unlikely Harry will be given the honour until his father the Prince of Wales becomes King of England.