The couple exchanged broad smiles as William, resplendent in a blue velvet cape and black velvet hat topped with ostrich feathers, passed her during the annual procession of knights at Windsor Castle.
Kate, 32, in a sky blue Christopher Kane coat and hat by Lock & Co, watched from a side porch with Sophie, Countess of Wessex, as their husbands walked down from Windsor Castle to the service at St. George's Chapel.
The Garter Day procession and service celebrates the oldest order of chivalry. The knights are made up of senior members of the royal family, former prime ministers and servants of the crown.
Kate and Sophie had arrived together and when William, 31, arrived alongside his dad, Prince Charles, he looked across at his wife who beamed broadly.
As Queen Elizabeth and Prince Philip followed, Kate made a deep curtsy.
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At the end of the service, Kate and William joined Charles, 65, in an open-topped landau carriage and passed cheering crowds as they rode back to the castle.
The service came after a heady weekend of royal events. The annual birthday celebration for Queen Elizabeth, or Trooping the Colour, happened Saturday, when the royals gathered to share a laugh and watch the soldiers parade and a enjoy a fly-past from the balcony at Buckingham Palace.
Last year Kate missed the Garter Day ceremony because she was heavily pregnant with George.
She first watched William at the ceremony in 2008 when he was installed as a Knight of the Garter.
It is an order founded more than 600 years ago by Edward III, who sought to institute a band of knights similar to those on King Arthur's round table.
Among the crowds of 7,000 specially invited members of the public watching the procession inside the castle grounds was royals fan Kaz Aston, 40, who runs the charity Monarchy Online.
"This is the seventh time I have been," she said. "It's got particularly exciting in the past few years with William and Kate. We missed her last year when she was pregnant but everyone was screaming loudly for Prince William. She looked lovely today, the pale blue is a lovely color."