Continuous gave trainer Aidan O'Brien a seventh St Leger victory by winning ahead of Frankie Dettori's mount Arrest and the King's horse Desert Hero.
The 3-1 victor galloped relentlessly to the line under Ryan Moore for an impressive victory at Doncaster.
Dettori was two and three quarter lengths behind in his final Classic before retirement later this year.
Desert Hero, owned by the King and Queen, stayed on for third with the Royal couple watching on.
Bred by Queen Elizabeth II, who died a year ago, a win for Desert Hero would have been the first Royal victory in a Classic since Dunfermline won for King Charles III's mother in 1977.
The King and Queen's presence added an extra sense of occasion to the race, and they were in the parade ring as the national anthem was played beforehand.
But Desert Hero, ridden by Tom Marquand for William Haggas, could not follow up his triumph at Royal Ascot in June.
Continuous forged to the front with two furlongs to go and stayed clear of 11-4 favourite Arrest, trained by John and Thady Gosden.
"He's a lot of class this horse, and he does stay and handles soft ground," said O'Brien.
The Irish trainer said there was a chance Continuous could head for Europe's richest race, the Prix de l'Arc de Triomphe, in a fortnight's time.