Kate, 42, stepped out ahead of the other royals to greet guests and performers, but the Prince of Wales and their children couldn't hide the joy on their faces as she returned outside Westminster Abbey when they arrived.
Prince William greeted his wife with a beaming smile and a cheerful "Hi," and tenderly touched his wife's arm before the Wales family placed their dedications on the Kindness Tree.
Watch the moment below…
WATCH: Prince William greets wife Kate with subtle PDA at Christmas concert
She was heard telling singer Paloma Faith: "I didn't know this year was going to be the year I've just had."
When Faith replied, "The unplanned", Kate said: "The unplanned, exactly. But I think lots of people this year have had such challenging times."
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This year's carol service was dedicated to those "who have shown love, kindness and empathy to others in their communities".
Prince George, 11, Princess Charlotte, nine, and Prince Louis, six, placed their messages on the Kindness Tree, with Louis writing a heartfelt message to his maternal grandparents, Carole and Michael Middleton.
It read: "Thank you to granny and grandpa because they have played games with me."
Carole, Michael and Kate's siblings, Pippa Matthews and James Middleton, were among the 1,600-strong congregation.
And it was a right royal turnout as the Duchess of Edinburgh, Princess Beatrice and Edoardo Mapelli Mozzi, Zara Tindall, the Prince and Princess Michael of Kent, Lord and Lady Frederick Windsor, Lady Gabriella Kingston, and the Duke and Duchess of Gloucester were also spotted among the guests.
25 December 2023 – Kate is last seen publicly on Christmas Day
9 January 2024 – Kate celebrates her 42nd birthday, with the King and Queen sharing a behind-the-scenes photo from the coronation to mark the day.
11 January 2024 – William returns to royal duties, meeting rugby heroes Rob Burrow and Kevin Sinfield at Headingley Stadium in Leeds.
17 January 2024 – Kensington Palace announces in a statement that Kate has undergone abdominal surgery at the London Clinic and that William has temporarily stepped back from royal duties to care for Kate and their children.
18 January 2024 – William visits Kate at the London Clinic.
26 January 2024 – King Charles visits Kate in hospital before undergoing treatment for an enlarged prostate.
29 January 2024 – Kate is discharged from hospital and returns home to Windsor to continue her recovery. King Charles also leaves the London Clinic on the same day after his treatment.
7 February 2024 – William resumes royal duties attending London's Air Ambulance Charity gala with Tom Cruise.
10 February 2024 – William and Kate spend February half-term with George, Charlotte and Louis at their Norfolk home, Anmer Hall. Kate's siblings are also pictured on holiday during the school holidays, with James Middleton and Alizee Thevenet flying to the Caribbean with baby Inigo, and Pippa Matthews and her family holidaying on the island of St Barts.
27 February 2024 – Kate said to be "doing well" as William pulls out of King Constantine memorial due to a "personal matter".
29 February 2024 – Kensington Palace addresses social media speculation, reiterating: "We were very clear from the outset that the Princess of Wales was out until after Easter."
10 March 2024 - A photograph of Kate with their three children at home in Windsor is released to mark Mother's Day and to thank the public for their "continued support".
11 March 2024 - Kate publicly apologises for the "confusion" around the photograph as she admitted she "occasionally experiments with editing," after news agencies pulled the image amid claims it had been digitally altered. The Princess is then seen in a car with William as she's driven to a private appointment in London.
19 March 2024 - Footage of William and Kate leaving the Windsor Farm Shop is shared by The Sun and TMZ in the first video sighting of the Princess since surgery.
22 March 2024 - Princess Kate confirms that she has been diagnosed with cancer in a moving video message and is undergoing preventative chemotherapy.
18 April 2024 - Prince William returns to royal duties after spending Easter holidays privately as a family.
10 May 2024 - Prince William says Kate is "doing well" as he visits the Isles of Scilly.
21 May 2024 - Princess Kate is "excited" as Kensington Palace shares update on her early years work with the release of her Business Taskforce's prioritising childhood report.
8 June 2024 - Princess Kate writes letter to Irish Guards to apologise for not being able to take part in the salute at the Colonel's Review.
14 June 2024 - In a personal statement, Kate shares she is making "good progress" and that her "treatment is ongoing and will be for a few more months".
15 June 2024 - Kate makes her first major public appearance since diagnosis at Trooping the Colour.
14 July 2024 - Kate watches the Men's final at Wimbledon with Princess Charlotte and Pippa Matthews.
11 August 2024 - William and Kate congratulate Team GB athletes in a video message after the Paris 2024 Olympics.
9 September 2024 - Kate confirms she's completed chemotherapy in moving video message.
10 October 2024 - Kate joins William on visit to bereaved families in Southport.
9/10 November 2024 - Princess attends Festival of Remembrance and Remembrance Day service at Cenotaph.
3 December 2024 - Kate greets Emir of Qatar alongside William during state visit.
6 December 2024 - The Princess of Wales hosts her Christmas carol service at Westminster Abbey.
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