King Charles III celebrates first winner in royal racing silks
The 10-3 shot was ridden by Ryan Moore and trained by Michael Stoute - the same combination behind Queen Elizabeth II's win with Estimate in the Gold Cup at Ascot in 2013.
"A winner for the King," said Stoute. "It is great to have done that for him - and he was very impressive."
The king's racing manager John Warren, said: "I'm delighted that the King and Queen Consort [Camilla] have had their first winner and they'll be delighted.
"I'm also absolutely thrilled for Sir Michael Stoute, who was probably the longest-standing trainer for Her Majesty, and for Ryan Moore, who has put in a big effort as well."
Just Fine, the sixth runner in the king's name, won Leicester's Premier Event Hire Venue Handicap by four-and-a-half lengths.
Charles III had his first runner in the royal purple, red and gold colours at Salisbury on Thursday, with Educator finishing second.
Charles has previously had winners in the colours he shared with Queen Consort Camilla when they were the Prince of Wales and Duchess of Cornwall.