In 1898, 29-year-old Keppel met Edward, Prince of Wales (later King Edward VII), the 56-year-old heir apparent to the throne. It was not long before she became one of Edward's many mistresses, despite a twenty-six-year age difference. The Prince immediately made her "La Favorita" and his semi-official mistress. Keppel lived at Pleasure House, East Sutton, Kent, where Edward visited her regularly; her husband conveniently left during the visits. Her relationship with Edward would last through his ascension to the throne in 1901 until his death in 1910. Keppel was one of the few people in Edward VII's circle who was able to smooth his strange mood swings. She was able to turn the cranky monarch into a happy man.
Edward's wife, Alexandra of Denmark, was fond of her and tolerant of the liaison. She preferred Keppel to Edward's previous mistress, Daisy Greville, Countess of Warwick, whom she disliked for being indiscreet when she showed off her position. Millicent Leveson-Gower, Duchess of Sutherland, Lady Warwick's half sister, stated that the prince was "a much pleasanter child since he changed mistresses".
Costume Ball, Bal Poudre at Warwick Castle in 1895
Through her royal association, Keppel became richer. The king let his friends like Sir Ernest Cassel create funds that would keep her financially secure. Instead of giving her money direct from the Privy Purse, the king gave Keppel some shares in a rubber company; these later gained her £50,000, the equivalent of around £7.5 million today. Edward let his own bankers and financial advisers manage her businesses. He also got her husband a good job with higher pay. According to Christopher Hibbert, "George cheerfully went to work for Sir Thomas Lipton, who obligingly found him employment at the prince's instigation." With her influence, Keppel also found her brother Archie a place in the royal household: Archie was Groom in Waiting for the final three years of King Edward VII's reign. Keppel later took care of him and his family.