The Duke and Duchess Cambridge attended a black-tie reception in aid of the National Memorial Arboretum Appeal at St James's Palace.
The Duke launched the appeal - which aims to raise £12 million to develop the Arboretum in Alrewas, Staffordshire, into a world-renowned centre for remembrance - and became its patron.
Their outing follwoed the news that the couple will be separated for six weeks in the New Year.
Prince William will be flown out to the Falkland Islands in the New Year to complete his development as a pilot.
The Prince and his wife will spend their first Christmas together before Flight Lieutenant Wales will depart on his six-week mission.
He will be the co-pilot of a Sea King helicopter that will be on call throughout the duration of his stay on the islands.