Prince William and Kate Middleton were joined by Prince Harry and Meghan Markle for a walkabout outside Windsor Castle.
The throngs of onlookers erupted into cheers and applause as the foursome strolled out of the gate together on Saturday.
Despite the strained relations between the couples, the joint appearance signaled a united front in the wake of Queen Elizabeth II's death at the age of 96 on Thursday.
The brothers were both clad in navy blue suits with dark ties while their wives wore black dresses. The four walked side-by-side as they viewed the piles of flowers, notes and tributes that had been left outside the gate by members of the public.
The Sussexes held hands and at one moment wrapped their arms around each other. Kate was seen gently patting William on the back during the outing.
William, 40, and Kate, 40, who recently received the titles of the Prince and Princess of Wales, and Prince Harry, 37, and Duchess Meghan, 41, then walked over to the barricades to greet the crowd of mourners. William and Harry were seen chatting at one point before going their separate ways to speak with their supporters.
Kensington Palace told People magazine that William had extended the invitation for Harry and Meghan to join him and Kate on the outing.
A source told the outlet that security were not aware that both couples would be making an appearance together.
Despite exiting outside the castle in separate cars, the foursome left together in a car that was driven by William.
Royal journalist Omid Scobie tweeted, "The Waleses had always been scheduled to greet well-wishers at Windsor Castle, but royal sources say the decision to invite the Sussexes was made in the eleventh hour.
"It is, without a doubt, a significant moment in the history of the relationship between the two brothers."