William donned a British Royal Air Force uniform for the solemn occasion. Harry flanked his brother on his left for the event, but did not wear a military uniform, as he is no longer a working member of the royal family.
As the royal family proceeded into Westminster Abbey for the funeral, the brothers were joined by their spouses, Kate, the Princess of Wales, and Meghan, the Duchess of Sussex. William and Kate's two older children Prince George and Princess Charlotte also walked in the funeral procession for their great-grandmother.
William and Harry reunited following the queen's death on Sept. 8, having escorted the late monarch's coffin from Buckingham Palace to Westminster Hall last week, and leading a standing vigil for the queen's coffin over the weekend as she lied in state at Westminster Hall.
Both William and Harry were close to the queen and now, as their father has succeeded their grandmother as king, their roles within the royal family will likely change.
"Now, it's about the king, their father, and rallying around the king," ABC News royal contributor Robert Jobson said Monday. "They really need to stand up and be by his side, to be his liege men, his wingmen at this moment and hopefully, this sense of tradition, this moment of national celebration and pride as well as mourning will help them realize that they've now got to both work together in a way that will help him."
"I think there's been many olive branches to Harry and I think that maybe he will look at this and think, 'I've got a role that's beyond just my family. I may have a role still to play with my father and my brother,'" Jobson added.
Prior to the queen's death, the two brothers had only reunited publicly a handful of times in the two years since Harry and Meghan stepped down from their roles as senior working royals.
One of the few times they appeared together, at their grandfather Prince Philip's funeral last year, William and Harry walked in the same row but were separated by their cousin, Peter Phillips.
Twice following the death of the queen, William and Harry have walked side-by-side, first at the procession escorting the queen's coffin to Westminster Hall and then on Monday in the procession moving the coffin to Westminster Abbey for the funeral.
Last week, the Sussexes joined William and Kate outside Windsor Castle to view tributes to the queen and speak with members of the public.
The appearance marked the first time the two couples, once called the "Fab Four" by royal watchers, had been seen together in public in over two years.
A representative for the prince of Wales told ABC News William had invited the Sussexes to join him and Kate.
After viewing tributes and speaking with members of the public, the four left together in a car driven by William.
The two families now have homes close to each other in Windsor.
William and Kate moved this summer from Kensington Palace to Adelaide Cottage on the grounds of Windsor Castle, which is also where Harry and Meghan's U.K. home, Frogmore Cottage, is located.
Both William and Harry shared touching tributes to the queen after her death, describing how much she meant to them and to their families.
"While I will grieve her loss, I also feel incredibly grateful. I have had the benefit of The Queen's wisdom and reassurance into my fifth decade," said William. "My wife has had twenty years of her guidance and support. My three children have got to spend holidays with her and create memories that will last their whole lives."
Harry issued a separate statement.
"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," he said. "I cherish these times shared with you, and the many other special moments in between."