The Mental Health Summit will see political figures, leading academics and policy-makers from around the world come together with one common goal: better mental health for all.
The Duke and Duchess viewed the work of contemporary fine artist Dario Vargas, who had painted an original piece for the event.
The former Ms Middleton studied Art History at St Andrew's University in Scotland - so we reckon she knows her stuff too.
"That's pretty much ruined the picture", William joked, via E! Kate and Will welcomed Prince Louis on April 23.More news: Nick Kyrgios rows with umpire during Shanghai Masters first-round exit
This saw the couple break out into laughter along with the crowd.
The Duke and Duchess had a chance to speak to young delegates at the summit about the challenges and issues they face.
Meanwhile, Meghan has also opted for the Prince of Wales check before, wearing an off-shoulder top by NY brand Theory during her visit to Cardiff in January.
The project's founder Dixon Chibanda said: "Bringing our Friendship Bench to London helps show leaders and campaigners how we can all empower communities to tackle the global gap between mental health problems and the treatments available".More news: Hubble gyroscope failure puts space telescope into safe mode
Middleton attended her first solo royal engagement since her son's birth last week, when she visited the Paddington Recreation Ground in Ewhurst, England to spend time with local school children.
The Queen of Outfit Repeating brought one of her classics out from the archive, a lilac Emilia Wickstead crepe dress featuring an a-line skirt, full-length sleeves and a sabrina neckline.
The Cambridges have two older children - Prince George, five, and Princess Charlotte, three.More news: Majorca Floods: Two Brits Reported Dead After Heavy Rain Sweeps Island