It's a tradition going back to 1901 when St Patrick's Day Shamrocks were first presented to the Irish Guards by Queen Alexandra.
Since then, the job has been handed down through the Royal family and now, it's Kate, the Duchess of Cambridge who has the honour.
Her husband William is the Colonel of the Regiment.
And while the shamrock is traditionally presented to honour the soldiers, who draw most of their recruits from northern Ireland, the regimental mascot gets one too....after some trial and error on this occasion.
After the traditional regimental photographs it was a short walk to the refreshment tent... and a glass of Guinness, but not for the Duchess.
She's eight months pregnant.