Catherine, 37, put on a stunning display in a stylish emerald green outfit, choosing a fitted Alexander McQueen coat coupled with a matching hat and simple black heels. The children's mental health activist further accessorized her ensemble with Kiki McDonough's diamond earrings with green tourmaline and amethyst.
William is the colonel of the Irish Guards, and wore the uniform for his marriage to Kate.
After watching the 350 soldiers march on Parade Square Kate was given a cheeky piece of parenting advice while cooing over a 12-week-old sleeping newborn.
The Royal Family have been presenting shamrocks to the Irish Guards - except during wartime - since 1901, when Queen Alexandra began the tradition.
The couple will later meet members of the regiment and attend a lunch.
During her pregnancy with Prince Louis, Middleton attended a St Patrick's Day event and chose to sip on water instead.
The shamrock's three heart-shaped leaves represent Ireland and the Holy Trinity.
Alongside its role serving in British conflicts overseas, the Irish Guards also serve in ceremonial and public duties at Buckingham Palace, Windsor Castle, St James's Palace and the Tower of London.
The royal mom presented shamrocks to Officers and Warrant Officers, and William, 36, saluted the marchers.
The royal was given a tour of the new brewery and met with apprentices from a program that aims to help people out of long-term unemployment by training them as chefs in Brains pubs.