For her appearance, Middleton wore a lavender Emilia Wickstead dress and carried a crocodile Aspinal of London handbag, accessorizing her ensemble with nude pumps, silver earrings and a matching necklace.
William and Kate have always been involved in mental health initiatives through their Heads Together campaign.
The summit saw political figures, academics and policymakers discuss issues surrounding mental health, with the event working to help countries work together to address said issues.
Eagle-eyed fans of Kate's style will recognise the frock from her and Prince William's royal tour of Germany and Poland previous year.
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.
They also took the time to meet with young people who have either had or are still dealing with mental health issues and met with representatives from a number of organisations running related campaigns and programs. At the summit, William and Kate visited a stand run by the non-profit organisation United for Global Mental Health, where they heard about the work of Dixon Chibanda, who started an initiative in Zimbabwe teaching elderly women the skills to offer therapy sessions on "friendship benches".
The couple is also preparing for another big event on Friday when William's cousin, Princess Eugenie, celebrates her Royal Wedding to fiance, Jack Brooksbank, at Windsor Castle.