Despite not joining his parents and uncle on Saturday, Prince George was acknowledged by the famous cycle race's organisers and gifted with his own mini yellow jersey, the item given to winners throughout the Tour de France.
Royal fans, who lined the Yorkshire streets to cheer on the cyclists, were also keen to talk about George, with one little girl asking where he was to which Kate replied "he's asleep in his bedroom".
The Duchess also gave an insight into what the little Prince enjoys playing with, telling one well-wisher that George is "interested in helicopters and anything with wheels."
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Duchess Kate cutting the ribbon to start the race
William, Kate and Harry saw the 22 teams begin the 2,272-mile trip, which will head through the UK, Belgium and Spain before finishing in Paris on 27 July.
For the occasion, the 32-year-old Duchess sported a vibrant green coat and nude heels while William and Harry wore smart blue trousers and navy suit jackets.
Following her appearance at Harewood House, Duchess Kate present the yellow jersey to German cyclist Marcel Kittel, who won the first leg of the race.
Kate also met athletes supported by SportsAid, the charity of which she is patron, at the finishing line in Harrogate.
William, Kate and Harry are all big supports of British cycling. They were there to cheer on Team GB's men's sprint team at the London Olympics in 2012, with the Duke and Duchess enthusiastically celebrating together as Sir Chris Hoy and his teammates won gold in a breath-taking race.