Click on photos below for gallery of Kate's visit.
The Duchess of Cambridge, who became patron of the charity in April this year, delighted crowds and schoolchildren as she arrived at the Clifford Chance office in Canary Wharf where the charity was hosting a forum.
Facing the rain, Kate emerged from her car and broke into a huge smile as she was presented with a bunch of flowers from a young schoolgirl, nine-year-old Tierney Potter.
The beautiful royal donned a Max Mara jacket and pleated Orla Kiely skirt, the same designer whose bird-printed coat she wore yesterday. Her brunette curls stayed neatly in place as she spoke to Tierney and tenderly clasped her hands.
"I felt nervous but excited," said the schoolgirl, when asked how she felt about meeting Kate. "It was a bit of an unusual school morning. She was nice and she was pretty."
The new mum was led into the office where she met the rest of Tierney's classmates from nearby St Edmund's Primary School and was greeted with a chorus of "hello".
Kate was treated to some singing by the group of schoolchildren and looked delighted at their performance.
"Have you been practising?" asked Kate, to which they unanimously replied, "Yes!"
"Thank you so much," added the Duchess.
During her visit, Kate listened to various guest speakers at the forum, starting with Place2Be chief executive Benita Refson. The forum was being held with the aim of increasing children's emotional strength and will.
"Your Royal Highness, we are delighted that you are joining us today," said Benita, as she addressed Kate and the rest of the audience. "You share our passionate belief that early intervention and children's wellbeing and mental health really does matter."
Seated front row, Kate listened attentively as the speakers covered a range of important issues, including cyber bullying, special needs, self-harm and preventing addiction.
Kate enjoyed her visit so much that she extended her stay by an hour to attend two more sessions on tackling addiction and self-harm.
As emphasised in the speeches, Place2Be helps 75,000 children in more than 200 schools. According to the charity, over two-thirds of those supported had shown improved emotional and mental wellbeing.
After the speeches, Kate met with the speakers and other industry experts, including head teachers and the founder of an addiction treatment centre, and took the opportunity to ask questions. As she ended her visit, Kate jumped back into her waiting car and waved at crowds who applauded her as she left.
The Duchess' visit comes just a day after she and her husband Prince William paid a visit to crime prevention charity Only Connect on Tuesday.