As well as celebrities, cancer survivors and others who have been affected by the disease were invited to the bash, which was raising money for the Royal Marsden Hospital.
The Queen gave Prince William special permission to use the castle's State Rooms for the evening, though guests hoping to bump into Kate Middleton were left disappointed, as the Duchess of Cambridge was at home looking after Prince George.
As he met Ralph Lauren, William joked: “I'm sorry my wife's not here, you probably would far rather you saw my wife.”
He later delivered a speech to his famous guests, all of whom are friends or supporters of the hospital, saying: “I am enormously grateful to Ralph Lauren for his generosity, and to all of you who so kindly support the hospital. You should know that what you do saves lives and gives hope to thousands of people.”
As well as the fashion designer, the Prince was photographed mingling with a number of his famous guests, and was even seen getting a laugh out of Kate Moss - an achievement if ever there was one.
He was also spotted speaking to 'Sherlock' star Benedict and the two looking like old friends as they chatted away at the fundraiser.
Check out the gallery below for all the pictures from the star-studded fundraiser...