Kate wore a teal Catherine Walker & Co coat dress with a beautiful brooch that matched her hat and heels. The royal couple wore shamrocks.
William's shamrocks were front-and-center on his military hat.
During the parade, William and Kate posed with those in the Irish Guards and gave speeches.
William previously held the position, but became the equivalent for the Welsh Guards to go with his Prince of Wales title.
William spoke about his pride in the regiment and how he will continue to watch them in the future.
The royals celebrated St. Patrick's Day by sipping a pint of Guinness beer.