Prince William reportedly felt "uncomfortable" greeting well-wishers at Windsor Castle without his younger brother, Prince Harry.
The Prince of Wales and the Duke of Sussex, with their wives Kate Middleton and Meghan Markle, viewed flowers left for Queen Elizabeth II outside the castle and talked to crowds gathered on the Long Walk on Saturday.
Well-wishers offered their condolences and passed them flowers as the "Fab Four" were reunited publicly for the first time since Harry and Meghan stepped down as senior royals and moved to the US.
Brothers William and Harry arrived together with their wives and spent about 40 minutes talking to people outside the castle.
It is understood the Prince of Wales did not want to go out in public to honour his grandmother's memory without his brother as he did not want Harry to feel excluded.
The Times reports that William was also upset by the focus on family tensions following the Queen's death.
There was speculation that the rift between the two brothers had continued as Harry travelled to Balmoral alone on Thursday and was the first member of the Royal Family to depart on Friday morning.
However, William invited the Sussexes to join him and his wife on the walkabout on Saturday - and Harry immediately agreed.
A spokesman for the Prince and Princess of Wales said: "The Prince of Wales invited the duke and duchess to join him and the Princess of Wales.
"The Prince of Wales thought it was an important show of unity at an incredibly difficult time for the family."
The walkabout was emotional and a royal expert said seeing the two brothers together is exactly what the late monarch would have wanted.
Ingrid Seward wrote in the Sunday Mirror: "It was a moment the nation had long been hoping for – the new Prince of Wales and his estranged brother Prince Harry putting their differences aside.
"When they jointly greeted the crowds outside Windsor Castle today, you knew they had put duty before themselves.
"It was just as Granny would have wanted, as she would have done. And it had to happen.
"People say grief brings people together and I agree it does. When a person who you both loved very much dies, you are united in grief and memories."
The two brothers also left together, with William driving the car and Catherine in the front with him, while Harry and Meghan sat in the back.
This morning, Prince Harry paid an emotional tribute to his late grandmother as he thanked her for her "sound advice" and "infectious smile".
Harry also described her as a "guiding compass" through her commitment to service and duty.
He said: "In celebrating the life of my grandmother, Her Majesty The Queen - and in mourning her loss - we are all reminded of the guiding compass she was to so many in her commitment to service and duty.
"She was globally admired and respected. Her unwavering grace and dignity remained true throughout her life and now her everlasting legacy.
"Let us echo the words she spoke after the passing of her husband, Prince Philip, words which can bring comfort to all of us now: 'Life, of course, consists of final partings as well as first meetings."'
He added: "Granny, while this final parting brings us great sadness, I am forever grateful for all of our first meetings-from my earliest childhood memories with you, to meeting you for the first time as my Commander-in-Chief, to the first moment you met my darling wife and hugged your beloved great-grandchildren.
"I cherish these times shared with you, and the many other special moments in between. You are already sorely missed, not just by us, but by the world over.
"And as it comes to first meetings, we now honour my father in his new role as King Charles III. Thank you for your commitment to service. Thank you for your sound advice.
"Thank you for your infectious smile. We, too, smile knowing that you and grandpa are reunited now, and both together in peace."
You can leave your tributes to Queen Elizabeth II here
