Her trip to meet inmates at HMP Send on Wednesday was part of the launch of her landmark national survey about the early years development of children, titled "five big questions on the under fives".
Kate met people who are receiving and have received help in their rehabilitation from The Forward Trust.
She had visited the prison in 2015 and was reacquainted with some of the women she met five years ago, who trace their history of offending and addiction back to troubled childhoods.
The duchess said to a group who have since been released from custody: "It really shocked me when I came here last time how early the challenges were that you face. How early you could take it back."
In the prison visits hall, sitting around a coffee table with mugs of tea and a cake, she chatted to five ex-offenders about their childhood traumas - problems with alcoholic or absent parents, family breakdown, domestic abuse - and how they believed it had triggered their offending.
She had similar conversations in a smaller room with four women currently serving sentences at the prison who told her stories about parents splitting up, drink, and drug addiction.
One current prisoner, Francesca, told Kate that she got on drugs and started offending after her loving parents split up.
At Send with the Forward Trust programme she has started to resolve many of her issues going back to childhood. “What they have done here has literally changed my life. It’s a miracle,” she said.
She added: “Coming to jail is one of the best things that has ever happened to me.”
The Duchess smiled and said: “It's so often I hear that. Why does it have to get to that point before people receive the help and support?"
She told the women: “I'm hugely passionate about trying to really help get into this crisis, trying to help provide that prevention mechanism and that support system in our communities.
"Particularly that support in the early years of life."
When she asked the earlier group of ex-offenders how much of their problems they put down to their childhood, at least three of the women - Hayley Wood, Kerrie Peters, and Kirsty Day - replied in unison: “All of it.”
Among the group was Julie Muir, who now works as head of recovery at The Forward Trust. She became addicted to drugs after family problems when she was a teenager.
“I spent my 21st birthday in prison. That was a wake up for me,” she said.
Asked why most people ended up committing crimes and going to jail, she replied: “Childhood trauma. It’s all to do with trauma in the early years - parents separating, domestic abuse, addiction, children not being given the right emotional tools,” she said.
“We need more support for parents, more counselling services.”
Benny Refson, trustee at The Forward Trust, called the duchess's interest in their work "huge", adding: "It's actually recognising the importance of the work, and the complex world that prisons are, and the uncomfortable world, and by her coming to these prisons it's giving value to the work that people do."
HMP Send is a resettlement prison housing an 80-bed unit that helps women with rehabilitation, building skills and confidence.
The women she met had been through a 12-step drug treatment created by The Forward Trust, which works in 25 prisons across Britain.