When asked how he was, Kate replied, "He is very well. He's sleeping at the moment."
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She also revealed that her mother Carole Middleton had been tasked with looking after the infant royal while she and William attended the event.
Many local residents, competitors and organisers had gathered to catch a glimpse of the new parents and the couple were presented with a variety of gifts.
The Duchess was given a heartfelt handmade butterfly-shaped decoration from three little girls.
Kate thanked the trio warmly and promised to share the gift with George when she got home as he loves bright colours.
The new parents were also given nappies. William laughed and thanked the present bearer, "We've got a lot of these. We can always do with some more."
Kate was positively radiant and relaxed as she made time to chat to the enthusiastic gathered crowds. She proved that she was every every bit the fashionable first-time mother in a floaty patterned top, black skinny jeans and her trademark LK Bennett wedges.
The spotty top, thought to be by high-street label Zara, is clearly one of Kate's favourites — she wore it in the early stages of her pregnancy when she went toboganning in Arosa, Switzerland.
With an affectionate nod to her rural surroundings, the 31-year-old completed the look with a light green tweed jacket, a Ralph Lauren number that she has worn on previous occasions.
Just five and a half weeks after the birth of her baby boy, Kate was back to her usual svelte figure. She wore her luscious brunette locks loose and kept her makeup to a minimum, allowing her natural glow to shine through.
The Duke of Cambridge rang the bell to begin the gruelling 135-mile Ring O'Fire ultra marathon and the new parents cheered the brave runners on their way.
The tough marathon follows the Anglesey Coastal Path around the island which William and Kate have called home since 2009.
It is thought that Friday's appearance could be one of their last on Anglesey before they leave when the Duke's tour of duty ends in September.