Sophie, the Countess of Wessex, stood at the viewing gallery while the Queen’s four children stood vigil by her coffin.
The silent vigil lasted for fifteen minutes, as members of the public walked by, having queued for hours to pay their respects.
Sophie looked mournful as she watched over her children, Lady Louise, 18 and James, Viscount Severn, 14.
The vigil comes ahead of Her Majesty’s State Funeral, which will be held on Monday.
Standing beside Sophie was Camilla, now Queen Consort, who watched as her husband King Charles III led the vigil.
Sophie’s husband Prince Edward, the Earl of Wessex, paid tribute to his “beloved mama” ahead of tonight’s vigil.
He added that the Royal Family has been “overwhelmed by the tide of emotion” following the Queen’s death.
The couple spoke with stewards and Crown Estate staff outside Windsor Castle as they viewed tributes to the late monarch.
The Countess placed bouquets given to her by the crowd with other floral tributes as Edward read some of the messages left for his mother.
Tomorrow the Queen’s eight grandchildren, including the Prince of Wales and the Duke of Sussex, will mount their own vigil to pay their respects to their grandmother.
Edward and Sophie’s children will also take part in the vigil on Saturday.
The Wessexes have largely endeavoured to give their children a relatively normal childhood, making their recent public appearances a rare occurence.
While Lady Louise has increased her appearances in recent years given her age, her younger brother James remains largely out of the spotlight.
His public appearance for tomorrow, therefore, represents a new departure for the young royal.
In a tribute released on Friday afternoon, the Earl of Wessex said the Queen’s death had left an “unimaginable void in all our lives”.
He said: “As a family, we have grown up learning to share our parents, especially our beloved mama, with the nation, her realms and the Commonwealth. While it has been lovely to have spent time saying our own farewell privately at Balmoral, it is now time to allow others to be able to say their farewell.
“We have been overwhelmed by the tide of emotion that has engulfed us and the sheer number of people who have gone out of their way to express their own love, admiration and respect to such a very special and unique person who was always there for us. And now, we are there for her, united in grief. Thank you for your support, you have no idea how much it means.
“The Queen’s passing has left an unimaginable void in all our lives. Sophie and I have taken huge pleasure in seeing our James and Louise enjoying the places and activities that their grandparents loved so much. Given that my mama let us spend so much time with her, I think she also rather enjoyed watching those passions blossom. Those times together, those happy memories, have now become massively precious to each and every one of us.”