The two sons of King Charles, once so close after the death of their mother Diana in a Paris car crash in 1997, have fallen out in recent years after Harry and Meghan gave up their royal titles to move to the United States.
The couple had been in Britain on a brief visit when the queen, Britain's longest reigning monarch, died on Thursday.
Like other senior members of the royal family, Harry rushed to be by the queen's side as she deteriorated at Balmoral Castle in Scotland. A spokesperson for the heir to the throne said William had extended an invitation for the Windsor walk.
A royal source described it as an important show of unity at an incredibly difficult time for the family. They emerged from the same car, all wearing black.
The two couples did not interact much during the 40-minute walkabout near one of the queen's favourite English homes, as they stopped to read messages left among the flowers and shake hands and chat with the thousands of people lining the Long Walk.
Royal observers had been looking for signs of a detente amid the pomp and ceremony of a period of national mourning and Elizabeth's state funeral.
Also, on Saturday, senior members of the Royal Family - including the queen's children, Princess Anne, Prince Andrew and Prince Edward - greeted well-wishers and read notes on floral tributes laid at Balmoral Castle in honour of the queen.
The royals emerged outside the Queen's Scottish home, where she died on Thursday (September 8) and her body still remains and was met by throngs of mourners who had gathered to pay their respects.
The state funeral for Queen Elizabeth will be held on Monday, Sept. 19, royal officials said on Saturday, as her son Charles was officially proclaimed Britain's new king in a colourful ceremony laden with pageantry and dating back centuries.
Charles, 73, immediately succeeded his mother but an Accession Council met at St James's - the most senior royal palace in the United Kingdom built for Henry VIII in the 1530s - to proclaim him as king on Saturday.
The council - formed of Privy Counsellors whose centuries-old role has been to advise the monarch - included his son and heir William, wife Camilla and Britain's new prime minister, Liz Truss, who signed the proclamation of his accession.
King Charles III hosted his first audience with senior politicians and members of the clergy, including the Archbishop of Canterbury Justin Welby.
The king appeared in good spirits, often sharing a joke with those he was seen chatting with.
Prime Minister Liz Truss, herself just days into her role as head of government, was seen introducing the new king to MPs.
Canada also proclaimed Charles its king on Saturday in a formal ceremony at the official residence in Ottawa of the governor general, the monarch's representative in Canada who performs the duties of the head of state on behalf of the crown.
Prime Minister Justin Trudeau, who earlier arrived with his youngest son Hadrien, and Governor General Mary Simon signed an order in council proclaiming the new sovereign after a Cabinet meeting. The table used for the event is the same table Queen Elizabeth used during her first official visit to Canada in October 1957.