Kate, who was to resume her royal duties since the birth of Prince George only by September 12, decided last minute to join William to launch the Ring O' Fire Coastal Ultra Marathon at the show in Anglesey, the island where the prince has been based as a Royal Air Force (RAF) search and rescue pilot.
Slim and relaxed, 31-year-old Kate, was dubbed a "yummy mummy" by the British press as she met some of the event's participants, as well as race volunteers, before getting proceedings underway.
According to local Welsh media reports, the event today was one of the final public engagements made by Prince William on Anglesey before his time their comes to an end.
The event, a 135-mile gruelling foot race around the rugged coast of Anglesey, is staged over three days and includes 13,695 feet of ascent.
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The 31-year-old Duke of Cambridge had said a public goodbye to Wales a fortnight ago as he confirmed his young family would be leaving their Anglesey home and moving "elsewhere" when his posting comes to an end this month.
He is widely expected to leave North Wales and return to London with Kate and baby George to take on more royal duties rather than stay in the service.
With the birth of George last month, the Duke and Duchess of Cambridge are now preparing to make Kensington Palace their family home.