It's been reported William invited the couple to join him and his wife as they greeted mourners grieving the death of the long-reigning monarch.
Although Harry and William have had a rocky relationship since the Duke and Duchess of Sussex departed from their royal duties and moved to the United States, a royal source noted it was "an important show of unity at an incredibly difficult time."
A second source stated security had not been aware that both couples would be there, indicating that it may have been a snap decision on William's part.
"It's such an extraordinary historical moment and also a deeply personal one for the family that you'd hope and think that all members of the family would unite and support [the King] especially," another source noted. "And perhaps some of those wounds can be healed in the process."
While speaking with onlookers, Harry reportedly commented that it's "a lonely place up there now without her" at Windsor.
This outing comes shortly after William shared his thoughts on his grandmother's passing in a heartfelt statement shared to the official Prince and Princess of Wales Twitter account.
"On Thursday, the world lost an extraordinary leader, whose commitment to the country, the Realms and the Commonwealth was absolute," he wrote in a message posted on Saturday, September 10. "So much will be said in the days ahead about the meaning of her historic reign."
"I, however, have lost a grandmother. And while I will grieve her loss, I also feel incredibly grateful," he continued. "I thank her for the kindness she showed my family and me. And I thank her on behalf of my generation for providing an example of service and dignity in public life that was from a different age, but always relevant to us all."