Catherine spent two weeks in hospital in January this year after she underwent major abdominal surgery. Two months later she announced in a video message that tests had revealed the presence of cancer and she was receiving preventative chemotherapy.
The 42-year-old is still undergoing treatment, but her improved health meant she was able to appear in public for the first time since December.
The Princess rode in a covered carriage with her three children to watch the "Trooping the Colour" on Saturday, an annual military parade in London to mark the official birthday of the British monarch, King Charles III.
Despite heavy rain showers, crowds lined the streets and waved and cheered as the carriage went past, outnumbering a small group of protesters holding placards calling for an end to the monarchy.
Charles rode in a separate carriage with Queen Camilla, followed by Prince William, Charles' heir and Catherine's husband, who was on horseback along with other senior members of the royal family.
She pointed out aspects of the event to her children Prince George, Princess Charlotte and Prince Louis, while Charles saluted passing troops from a covered dais on the parade ground itself.
The King and Queen, as well as William, Kate and other members of the royal family later returned to Buckingham Palace to watch a military flypast from the balcony. Kate smiled broadly and waved.
In a rare personal written message on Friday, she said she was making good progress but was "not out of the woods". She said she was looking forward to attending Saturday's parade and hoped to join a few public engagements over the summer.
Her office, Kensington Palace, has declined to give any details about the type of cancer or about her treatment, other than to say the preventative chemotherapy had begun in February.
'FABULOUS'
Before the parade, onlookers said they were looking forward to seeing the Princess, also known as Kate.
"I know it's the King's birthday, but all eyes will be on Kate because we haven't seen her for a while," said John Loughrey, who was wearing a T-shirt with pictures of Kate on it.
Another onlooker, Anne Gaely, who was carrying a life-size cardboard cutout of the Princess, said she was relieved that she would be there.
"We've all been very, very worried and uneasy about it, but now this is going to be joyful, euphoric and fabulous," she said.
In her message on Friday, Catherine said she had been "blown away" by thousands of kind messages from across the globe, which had made a world of difference to her and William.
"I am making good progress, but as anyone going through chemotherapy will know, there are good days and bad days," her statement said.
Her illness has coincided with that of Charles, 75, who has also been undergoing treatment for cancer.
He returned to public duties in April, and has remained busy, although his diary commitments are being limited to minimise risks to his recovery.
