The parade had a march-past and people handed out sprigs of shamrock.
Kate's first parade as colonel and her recent official visit to the 1st Battalion Irish Guards are both important events.
The King made Kate Colonel of the Irish Guards, Prince Charles and Queen Camilla colonels of other guard battalions.
Kate visited the battalion and learned about their efforts.
The day ended with a trip to the dining hall for the junior ranks and an official picture.
Kate wore a teal coat dress, a fascinator that matched, heels, and a shamrock pin made of gold.