During her maternity leave, Kate made a handful of public appearances largely for family events like brother-in-law Prince Harry's wedding to Meghan Markle and to mark the annual "Trooping the Colour" at Buckingham Palace.
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.
The young royals have been strong advocates for mental health, setting up the Heads Together campaign in 2016 through their official charity, The Royal Foundation.
The Duchess of Cambridge was all smiles as she arrived at County Hall (Victoria Jones/PA) Eagle-eyed royal watchers noticed she was wearing an Emilia Wickstead dress and Webb jewellery (Victoria Jones/PA) The duke and duchess were greeted by Health Secretary Matt Hancock (Victoria Jones/PA) The royals chat to dignitaries.
One of the policies up for discussion at the summit is a declaration committing to putting mental health on an equal footing with physical health.
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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.
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'.
Both wore outfits we've seen previously, with Prince William choosing a classic black and white suit and Kate choosing a long-sleeved lilac dress by designer Emilia Wickstead, which she wore during her royal tour of Germany in 2017.
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.
The Cambridges have two older children - Prince George, five, and Princess Charlotte, three.