She paired the dress with new, floral hooped statement earrings from the label and teamed the frock with deep purple accessories, including a waist cinching belt and plum velvet pumps from Jimmy Choo.
Another policy up for discussion is a declaration committing to putting mental health on an equal footing with physical health.
Celebrity stylist Rochelle White told FEMAIL: 'Kate's latest look is something different from her usual style.
The Duke and Duchess had a chance to speak to young delegates at the summit about the challenges and issues they face.
She also met with curators during her visit.
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".
She added her own artistic touch to an original piece by Dairo Vargas, a contemporary fine artist from Columbia, who created a painting inspired by the day's events.
The duchess first showed off the Emilia Wickstead dress on a trip to Germany previous year.
The couple, along with Prince Harry, have been credited with helping break down the stigma surrounding mental health by encouraging open conversations about it. He has since been joined by wife Meghan Markle at mental health charity events.
The couple are expected to attend Princess Eugenie and Jack Brooksbank's wedding on Friday 12 October.