The Duchess dazzled in the Queen Mother's lotus flower diamond tiara and chose a white gown by one of her favourite designers, Alexander McQueen.
It is the first time the 31-year-old has worn a tiara since her 2011 wedding. The delicate diamond and pearl design features fanned motifs and a showstopping central top pearl.
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It is the same crown worn by Princess Margaret to the same event 55 years ago in 1958 — click on photo above to see it.
The fashionista royal complemented the tiara, believed to have been loaned to her by the Queen, with a pair of diamond drop earrings. She wore her glossy brunette locks in a chic demi chignon not disimilar to her wedding day hairstyle.
The new parents arrived at the glittering annual Diplomatic Reception hosted by the Queen in a Bentley.
The event sees the 87-year-old monarch invite the 172 foreign ambassadors based in the capital to her official residence.
Her Majesty meets each member of the London Diplomatic Community at the largest event on the palace's calendar.
Royal commentators have observed that William's attendance at the event is preparing for his future role as King.
The Tuesday evening event is just one of the many engagements William and Kate have scheduled this week.
On Thursday night they will attend the royal premiere of Mandela: Long Walk to Freedom in London's Leicester Square.
On Friday Prince William will present medals Irish Guard soldiers in Aldershot while Kate visits youngsters at Shooting Star House Children's Hospice in Hampton.
Sunday will see William attend the British Military Tournament at Earls Court Exhibition Centre.
The event will raise funds for the three national charities of the National Armed Forces: ABF The Soldiers’ Charity, The Royal Navy & Royal Marines Charity and The Royal Air Force Benevolent Fund.
On Thursday 11 December the Duke and Duchess will attend the screening of David Attenborough's Natural History Museum Alive 3D.
The couple will meet Sir David Attenborough and other attendees before watching the film which is due to be broadcast on Sky next year.