She also sported a £650 felt burgundy headband with a black felt bow by milliner Jane Taylor.
Her Majesty was also joined by other members of her family at the service, including Princess Beatrice and newlyweds Princess Eugenie and Jack Brooksbank.
According to a German tradition introduced by Prince Albert, the Royal Family exchange gifts, which are then laid out on tables to be opened on Christmas Day.
The Duchess teams her Catherine Walker coat with heels and a "halo" headband. Well-wishers brought bouquets of flowers for the family and wished them a Merry Christmas as they walked by.
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Her classic look appears to be channeling from style icons like Katharine Hepburn and Ginger Rogers with defined shoulders and large collars.
British tabloids have been dominated by gossipy stories claiming that Kate, 36, and Meghan, 37, don't get along, or that Will, 36, and Harry, 34, are on the outs, or that relations are so bad that Harry and Meghan are moving away from the Cambridges at Kensington Palace to live in Windsor.
Meghan reportedly left Kate in tears over her "stressful" demands for Princess Charlotte's bridesmaid dress, at a fitting for her royal wedding.
Most of the royals were in attendance, except for the Duke of Edinburgh and the Duchess of Cornwall.
If there was any hint of the rumoured rift between them, it certainly didn't show as they laughed and chatted together.
Meghan, who is around six months pregnant, wore a navy coat and knee-high boots by British fashion designer Victoria Beckham while Harry donned a suit with a light blue tie.