'You are allowed to go anywhere, ' encouraged Kate, who was wearing a £1,500 bespoke lavender Emilia Wickstead dress - previously seen on the couple's 2017 tour to Germany - with £510 Gianvito Rossi nude heels.
The duke and duchess were attending the first Global Ministerial Mental Health Summit in London, which is being hosted by the Government and the Organisation for Economic Co-operation and Development.
William and Kate have always been involved in mental health initiatives through their Heads Together campaign.
Tuesday's summit focused on political figures, leading academics and policy-makers from around the world coming together with the common goal of better mental health for all.
They also heard presentations on two case studies of active mental health programmes from Slovenia and the United States of America and visited an exhibition area.More news: Trump apologises to Kavanaugh for bitter campaign, United States News & Top Stories
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While viewing the painting, the artist invited the royals to add to the piece, with Prince William nominating his wife as "the arty one", owing to her art history degree.
The summit is being held at London's County Hall on the South Bank.
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 couple also learned about Friendship Bench, part of an innovative programme from Zimbabwe, which has taught elderly women the skills to become trained 'community grandmothers'.
Let's hope Prince George, Princess Charlotte and Prince Louis are all in bed, because mom is out on the town!
The couple, along with Prince Harry, have been credited with helping break down the stigma surrounding mental health by encouraging open conversations about it.