Kate, whose mother is Goldie Hawn, explained how her brush with royalty came about.
"Prince William and Prince Harry actually stayed at our house in Colorado when they were kids," the How To Lose A Guy in 10 Days actress told Jay Leno on the Tonight Show on Wednesday.
"We were all in LA and they were staying with their mum at the time and we get this phone call at the house."
"Sure enough, I think it was Prince William calling my dad to ask if he could use the ATVs, the four-wheeler things."
First-time dad William's telephone manner clearly impressed Kate's father Kurt who said "that's a really well-raised boy" when he hung up.
Although mother-of-two Kate said she had never met any of the royal family, she followed the arrival of Prince William and Kate Middleton's first-born son Prince George.
"That was insane, actually. You can't help but follow it because it was on every news channel," she said.
"That's all you saw. That was the longest labour on news. It literally was full coverage."
The Golden Globe winner was in London in July filming scenes of her upcoming thriller alongside James Franco.
It was a family affair for Kate who was joined on set by her fiancé Matt Bellamy and her two children Ryder and Bingham.