The Duchess of Cambridge sipped on a pint of Guinness to traditionally mark St Patrick's Day.
She toasted the pint with her husband the Duke of Cambridge after a parade of the Irish Guards.
Prince William attended the event in his role as colonel of the 1st Battalion Irish Guards, an infantry unit who gather on St Patrick's Day for the ceremonial celebration.
Led by a regimental band and their mascot Irish wolfhound Domhnall, 450 soldiers marched on Parade Square at the Cavalry Barracks, Hounslow, west London.
During the parade Kate, wearing a dark green Catherine Walker coat, presented sprigs of shamrock to officers.
The royal couple then met some of the soldiers and their families for lunch in the welfare centre of the barracks where the longest-serving Guardsman in the battalion proposed a toast to and thanked the Duchess for presenting the shamrocks.
Lt Col Alex Turner said: "He (William) is our first royal colonel and so he brought a youthful vitality to the role which in the past has been a bit more stately."
The Duke showed he could relate to some of the children present when he recognised an episode of Peppa Pig which a toddler was watching.
Jo Fallon, who attended the parade, said William recognised the cartoon her son Zach was looking at on a phone.
"They (Kate and William) were very down to earth and very friendly," she added.
William wore an Irish Guards frock coat to mark his rank as colonel, and as he spoke with soldiers he asked about their training and careers.
Lt Col Alex Turner said: "A lot of the men have served in Afghanistan and Iraq, and we have been in the jungle in Belize.
"We are jungle specialists."
The Irish Guards are set to undergo further jungle training in Thailand.
When one soldier told William this, he laughed and said: "Enjoy Thailand, I can't wait to hear stories of Thailand!"