She wore a bespoke Catherine Walker coat dress, matching Gianvito Rossi pumps, and a Jane Taylor London hat.
The brooch belongs to the Irish Guards and is loaned out to royal ladies associated with the regiment.
Its origin is unclear, but it was reportedly designed by Cartier and gifted to Princess Mary by the regiment in 1961.
The regiment bought the brooch after Princess Mary's death for the Queen Mother and Princess Anne.
Princess Catherine first wore the brooch in 2011 during a presentation of service medals.
Princess Catherine looked stunning and elegant in her outfit and the brooch added a touch of royal history to her ensemble.