The Princess of Wales was seen publicly for the first time hours after issuing a public apology for editing a family photograph taken with her children, that was released on Mother's Day.
Kate, 42, who is recovering from abdominal surgery, was pictured in the car with her husband, the Prince of Wales, as they left Windsor in a black Land Rover.
The Princess was seen in the back seat of the vehicle with Prince William as they travelled from Adelaide Cottage to London, where William, 41, joined his stepmother Queen Camilla to head up the senior royals at the Commonwealth Day service at Westminster Abbey.
It's understood that Kate did not join him as she had a private appointment. The Princess is not expecting to return to public duties until after Easter.
The Duke and Duchess of Edinburgh, the Princess Royal, the Duke and Duchess of Gloucester and the Duke of Kent were among the royals in attendance.
William was seen chatting and smiling with Queen Camilla at the start of the service as the royals stood for the national anthem.
The Queen, 76, arrived at the Abbey moments after her stepson and looked beautiful in a turquoise coat with a matching hat, and two heart-shaped brooches.
Meanwhile, the Princess Royal, 73, was accompanied by her new lady-in-waiting and her daughter Zara Tindall's best friend, Dolly Maude. The appointments were confirmed earlier this year in the Court Circular.
Anne looked elegant in a red embroidered coat over a teal silk dress with a mint-coloured hat for the service.
The Duke and Duchess of Edinburgh were all smiles as they arrived at Westminster Abbey, with the event taking place just one day after Prince Edward's 60th birthday.
Sophie, 59, wowed in a flared coat and a trilby hat as Edward, 60, looked smart in a charcoal-grey suit.
Charles, 75, who is currently undergoing cancer treatment, pre-recorded a message in which he pledged to continue to serve the Commonwealth "to the best of my ability".
His address highlighted how "diversity" was the Commonwealth’s core "strength", and the union of 56 member countries was a “precious source of strength, inspiration, and pride" to him.
"In recent weeks, I have been most deeply touched by your wonderfully kind and thoughtful good wishes for my health and, in return, can only continue to serve you, to the best of my ability, throughout the Commonwealth,” the King said in his video message, which will feature in the service.
He added: "My belief in our shared endeavours and in the potential of our people remains as sure and strong as it has ever been.
"I have no doubt that we will continue to support one another across the Commonwealth as, together, we continue this vital journey."
Among the 2,000 guests were Commonwealth Secretary-General Baroness Scotland, the president of Malta, foreign ministers, high commissioners, senior politicians and dignitaries from across the Commonwealth, faith leaders and schoolchildren and young people from throughout the UK.
The service, which marked the 75th anniversary of the family of nations, featured performers from across the Commonwealth including a dance troupe from Uganda that delighted the royals.
The front row of Camilla, William, Edward, Sophie and Anne beamed as the youngsters, who featured on Britain’s Got Talent, twirled and jumped.
The Queen later attended the annual Commonwealth Day Reception at Marlborough House in London, to meet foreign ministers, UK parliamentarians and high commissioners.