Appearing on a balcony of the Foreign and Commonwealth office, Kate wore a structured black Alexandra McQueen coat paired with pearl-drop earrings.
The coat was adorned with several red poppies, and her hair was covered by a black hat.
The women watched from the balcony as Prince Charles laid a wreath at the Cenotaph in memory of Britain’s fallen soldiers.
The event was supposed to mark the Queen’s first public appearance after doctors advised her to rest following an overnight stay in hospital last month.
Earlier on Sunday, Buckingham Palace announced that the monarch would not be attending because she has sprained her back.
The palace said she had made the decision with “great regret”.
“Her Majesty is disappointed that she will miss the service,” a statement said.
“As in previous years, a wreath will be laid on Her Majesty’s behalf by the Prince of Wales.”
On Thursday, 11 November, Charles reassured members of the public in Brixton, London, that his mother is in good health.
The heir apparent was meeting young people helped by his charity, the Prince’s Trust, at a NatWest branch when one person asked him: “Prince Charles, how is your mother?”
Patting the man on the arm, the royal replied: “She’s all right, thank you.”
The Queen spent a night in hospital in October to undergo “preliminary checks”, Buckingham Palace said at the time.
Following the visit, she has cancelled a number of scheduled appearances such as at Cop26 and a visit to Northern Ireland.
She has still been carrying out some light duties and recently returned to Windsor Castle following a weekend stay at Sandringham.