William, 31, is the hospital's president, a position once held by his mother Princess Diana. Fashion designer Ralph Lauren announced at the event he is funding an entire new breast research facility at the hospital.
While Helena Bonham Carter, Cate Blanchett, Benedict Cumberbatch and Kate Moss were among the gala's guests, another star was not. William's wife Kate, 32, was busy looking after Prince George, 9 months.
When William was introduced to Lauren in the castle's Grand Reception Room, he apologized for Kate's not being there.
"I'm sorry my wife's not here," he said. "You probably would far rather [see] my wife."
"Not at all," the designer replied.
Lauren, 74, and his wife Ricky headed over from the United States for the launch of the Ralph Lauren Breast Cancer Centre. Lauren, who had a benign brain tumor removed 25 years ago, has donated millions to cancer research around the world over the past 20 years.
"I am enormously grateful to Ralph Lauren for his generosity and to all of you who so kindly support the hospital," William said during his speech. "You should know that what you do saves lives and gives hope to thousands of people."
William became president of the hospital in 2007. Last November, he watched some surgical procedures in one of its operating theaters.
He also supports the work of the Royal Marsden Cancer Charity, a member of the Charities Forum of which the Duke and Duchess of Cambridge and Prince Harry are patrons.