The father-of-three also touched her arm and Kate reciprocated the loving gesture.
If that wasn't enough to bring a smile to our faces, Prince William guided Kate to the car with an arm on her back - a protective pose in front of the camera flashes.
During the visit, Kate and William chatted to the mental health charity's volunteers and campaigners.
The new organisation's latest venture, Shout, famously marked the Sussexes and Cambridges first joint venture since splitting royal households.
The couples pledged to donate £3 million to the charity endeavour from the Royal Foundation - the biggest investment in a single royal project to date.
"We are incredibly excited to be launching this service, knowing it has the potential to reach thousands of vulnerable people every day," a statement from the foursome read. "Over the last few months Shout has started working quietly behind the scenes. We have all been able to see the service working up close and are so excited for its future."
For the outing, the Duchess of Cambridge braved the capital's chill in a pair of berry-hued Joseph trousers and a Prince of Wales check blazer by go-to brand Smythe.
A pair of black Gianvito Rossi block heels and Mappin & Webb drop earrings completed the winter-ready ensemble.