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.
William and Kate have always been involved in mental health initiatives through their Heads Together campaign.
During the event, William aimed a good-natured ribbing at his wife after Middleton, who studied art history at the University of St. Andrews in Scotland, added a brushstroke to artist Dairo Vargas' painting.
Catherine's dress was paired with plum, velvet Jimmy Choo heels while her gold earrings also by Erdem matched her simple clutch.
She also met with curators during her visit.
Kate returned to royal duty last week after being on maternity leave since the birth of Prince Louis in April. The initiative was started by Dixon Chibanda and teaches the women to become lay health workers.More news: Dianne Feinstein gets 'lock her up' chant at Donald Trump rally
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The Erdem dress worn by the Duchess last night was originally shown on the catwalk as part of the British brand's SS18 collection inspired by the Queen's 1950s wardrobe.
It's been said before that the mum of three has a keen interest in photography and has taken some of the official royal portraits of her children herself.
The couple, along with Prince Harry, have been credited with helping break down the stigma surrounding mental health by encouraging open conversations about it.
The couple are expected to attend Princess Eugenie and Jack Brooksbank's wedding on Friday 12 October.