Meghan Markle and Kate Middleton travelled in separate cars to Westminster Hall, where the Queen's body lies in state, before putting on a rare show of unity.
The royal pair, who have a frosty relationship at best, were both driven from Buckingham Palace to the 1000-year-old hall behind the Queen's coffin.
Meghan travelled with Sophie, Countess of Wessex, whereas Kate went with the Queen Consort.
The decision to travel separately came in stark comparison to feuding brothers Prince William and Prince Harry, who set their differences aside to walk side-by-side behind their father, King Charles.
Meghan was later seen standing alongside Kate - the new Princess of Wales - at the short service at Westminster Hall, with William and Harry next to them.
The Duchess of Sussex, 41, wore a set of pearl earrings given to her shortly after she married Harry in 2018. She wore the same jewellery for her only solo engagement with the Queen in June that year.
Kate, meanwhile, sported a brooch once worn by the Queen. She paired it with diamond and pearl drop earrings from Princess Diana's collection.
Harry and Meghan left the service together, bowing towards the Queen's coffin as they departed.
Other than their appearance together outside Windsor Castle - where they met wellwishers and looked at tributes left by the public - on Sunday, the Waleses and Sussexes have not been seen together since March 2020, when they attended that year's Commonwealth Day service.
The rare joint engagement outside Windsor Castle was nearly derailed by tense negotiations, which delayed the former Fab Four's arrival by 45 minutes.
Charles led the royal mourners as the Queen's body made its way from Buckingham Palace to the Palace of Westminster on Wednesday, where it will lie in state for four days.
Alongside him were Prince Andrew, Princess Anne and Prince Edward.
Also taking part in the procession were Anne's son, Peter Phillips, her husband, Vice- Admiral Sir Tim Laurence, the Duke of Gloucester and the Earl of Snowdon.
Zara and Mike Tindall, Princess Beatrice's husband Edoardo Mapelli Mozzi and Princess Eugenie's husband Jack Brooksbank joined them. Prince Andrew's ex-wife Sarah Ferguson was not invited.
Andrew and Harry were wearing morning suits, rather than military regalia, as they are no longer working royals. They did, however, wear their medals.
Members of the public - who are massed in their tens of thousands - were seen openly weeping as the cortege passed them by, while others applauded the new King.
The gun carriage carrying the Queen's coffin arrived at Westminster Hall just after 3pm.
She will now lie in state for the next four days, following a vigil attended by members of the royal family this afternoon.
Following the conclusion of the procession from Buckingham Palace, the guards carefully removed her coffin from the carriage outside the Hall and carried it inside in unison.
Sitting atop the coffin, which is draped in the yellow and red Royal Standard flag, is the Imperial State Crown upon a purple velvet cushion.
The crown is made of gold and set with almost 3000 diamonds, 17 sapphires, 11 emeralds, 269 pearls and four rubies.
After entering the Hall, the coffin was placed upon the catafalque by the Bearer Party.
As the choir sang, the King, along with members of the royal family, stood in formation behind the Queen.
The service for the Queen got underway shortly after her arrival at Westminster, led by the Archbishop of Canterbury, Justin Welby, and the Dean of Westminster, Dr David Hoyle.
It began with a reading from the Book of John, as well as the Lord's Prayer.
During the service, the Cross of Westminster was ceremoniously placed at the head of the coffin.
It features an inscription of the words by Isaiah 2:4: "Nation shall not lift up sword against nation. Neither shall they learn war anymore."
Just before 3.30pm, the royal party exited the Hall as the service wrapped up.
The new King, with Camilla by his side, paused at the door to share a private moment with the Archbishop of Canterbury, during which he appeared to offer gratitude for the ceremony.
The Archbishop responded with a bow of his head, saying "thank you" back to the monarch.
With the family having departed, Westminster Hall is now allowing the first members of the public in to pay their respects to the Queen.
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