Kate looked stunning in a cream coat dress with a pleated skirt and Peter Pan collar by Alexander McQueen. She accessorised with a matching hat with dusty pink roses by Jane Taylor, her trusty LK Bennett heels, clutch bag and a pair of pearl drop earrings.
Prince William and Prince Harry, meanwhile, were handsome in navy blue suits. William also wore the gold and silver Jubilee medals given to him by the Queen.
The royal trio were joined by 500 guests, including Prime Minister David Cameron, Irish President Michael D Higgins, King Philppe and Queen Mathilde of Belgium and German President Joachim Gauck for the twilight ceremony.
Earlier in the day, Prince Harry inspected a parade of troops in Folkestone and officially opened a Memorial Arch in dedication to the veterans of the Great War, before flying to Belgium to join his brother and sister-in-law.
Meanwhile, the Duke and Duchess of Cambridge were welcomed by Belgium's King Philippe and Queen Mathilde at the Saint-Laurent Abbey in Liege on Monday morning, as they marked 100 years since Britain's entry into World War I.
The royals made their way to the Cointe Inter-allied Memorial for a service, where William laid a poppy wreath at the British memorial.
Attendees, including more than 50 heads of state – among them France's president Francois Hollande and German president Joachim Gauck – took part in a minute of silence.
Prince William, Kate, Prince Harry with Nicolas Martin, acting mayor of Mons
Prince William made a speech, in which he paid tribute to Belgium. "Your Majesties, if I may say so, Belgium's steadfast remembrance of your war dead, and ours, is a great credit to your nation," he said.
"On behalf of Her Majesty Queen Elizabeth, head of the Commonwealth, thank you for the honour you do us all."
He went on to quote British nurse Edith Cavell, "who saved soldiers from each side. On the night before she faced a German firing squad, she said: 'I must have no hatred or bitterness towards anyone".
William and Harry's father, Prince Charles, also took part in the commemorations. Dressed in the uniform of a British admiral of the fleet, he attended a Service of Remembrance at Glasgow Cathedral.