With experience, people close to her say, has come a clarity and sure-footedness that no one can miss. The Duchess is clearer in how she wants to work, brimming with ideas on what she wants to say, and steadfast in her decision to put the wellbeing of her children at the heart of it all. Characteristically "cool and calm" about her milestone birthday, the Duchess wants no fuss, says one who knows her.
Instead, an official photograph will mark the occasion; symbolic of the Duchess' acceptance of her role in public life and willingness to share her family's joy with those who take an interest.
"Do women usually love everyone talking about how they're turning 40?" a source wondered.
"It probably feels a bit weird on an individual level, but it's nice that people want to celebrate and she rolls with it."
Those around the Duchess are united in their belief that the coming decade will see the careful planning of her lifelong work take root.
With the physically demanding baby years behind her, and the prospect of parenting teenage princes and princesses creeping closer, the Duchess has been able to plot her "legacy projects" with redoubled precision.
What's more, the idea that the Duchess will come into her own in her 40s is not without precedent. Queen Elizabeth, the Queen Mother, through the unhappy circumstances of the Second World War, found herself embraced by the nation as never before as she became the face of Blitz defiance, jollying along as one of her people. The Queen herself, having had her youngest child at 38, was similarly returning to the work that had never really slowed down, with her 40s seeing her inaugural walkabout on an overseas tour and an opening-up of the royal family's private life by inviting television cameras into their home.
The Duchess' late mother-in-law, of course, did not reach the milestone of 40. Diana's title of Princess of Wales has now been out of use for 25 years, left alone by the woman who could technically use it - the Duchess of Cornwall - out of respect and the strong public emotion it still inspires. There will likely be no such qualms when it comes to William's wife. Prince Charles technically has to invest his eldest son with the title when he eventually becomes king.
"It's hard to see it not happening, says one royal source. "I'm sure it'll be a bit of a bittersweet moment. This is her [Diana's] son and his wife, so in some ways it will be coming full circle - but it will also be a poignant reminder of what we all lost."
The Duchess, naturally, has been endlessly compared to Diana. Her wardrobe, the jewellery Prince William has given her from his mother's collection, even her recent star turn on the piano at the Westminster Abbey carol service - all have led to whispers of a new "People's Princess". But while some of it is superficial, those close to the couple say there is much more substance to the comparison if one chooses to look for it.
"They want to really listen to what people have to say, spend time with them, and keep those relationships going," says one insider. "The empathy that Diana showed to people changed the way the royal family communicated, and they are taking that on."
"It's the golden thread running through all [Catherine's] work now," said an aide.
"Human connection. She sees her role really as bringing people together."
Beyond the couple's work in front of the cameras, it is said, the Duchess regularly writes and makes telephone calls to the people she has met. And while she is often portrayed as being in a supporting role, she increasingly has firm ideas of her own about what she feels is the right thing to do.
In March, she broke from the usual cautious palace approach to attend a vigil for murdered woman Sarah Everard, having already written to her grieving family. When she struck up a friendship with pensioner Len Gardner, who had experienced loneliness in lockdown as he cared for his wife with Alzheimer's, she took careful note of his love of Italian cooking before sending him a pasta maker and then-hard-to-source flour.
"The conversations she has with people means that she is very intuitive about the public mood," says one familiar with her working style. "She likes private meetings with in-depth conversations, where she can really listen to what they are going through."
So far, that instinct has seen her shine through the pandemic, hosting a carol concert for the unsung heroes of lockdown, and a photography exhibition inviting anyone to contribute their defining images of this unique period of history.
The model for her work over the next decade is clear: an all-encompassing focus on the key topic of early childhood development, which she believes can be her lasting legacy. At the centre of it all, says everyone around the Duchess, are her children. Prince George, Princess Charlotte and Prince Louis have the best opportunity of any royal generation to lead happy, balanced and mentally healthy lives under the watchful eye of parents who are happy to share their joy with the world - but draw the line there.
While the Princess of Wales broke the mould in allowing Princes William and Harry to lark around in front of the cameras, the Cambridges have taken the best of that approach, but seized control of media access to keep the excesses of the paparazzi at bay.
A proposed move to Windsor, as eight-year-old Prince George looks towards his new school, is under discussion, in order to balance work and family life in a way that no royal has managed before.
"It's important for them to be successful parents," said a source.
The Duchess, another insider noted, has grown into her public role alongside the Duke over the past decade.
"They have found their niches, they enjoy what they do and are happy in their work. They're proud of the projects and want to make a contribution to society in the way that they can, here and internationally as well."
Tomorrow, fittingly, the Duchess will be celebrating her 40th birthday at home with her family. No doubt there will be homemade cards, a crumb-strewn breakfast made by eager children, and a moment of reflection from the ever-loyal Carole and Michael Middleton who have watched their little girl grow up.
By her next milestone birthday, in all likelihood Kate will be a princess in her own right. The nature of that role, and her place in the history books, is hers to define.
