Dominant at 10 furlongs this season, Ballydoyle's 4-year-old Danehill colt had yet to prove his staying power over the longer distance, and it appeared he might come up short in the $1.7 million classic after taking command.
With Johnny Murtagh aboard the 4-6 choice, Duke of Marmalade had driven well clear with a furlong left to run, but 14-1 Papal Bull, rallying on the outside after lagging for most of the trip, came on strong under Olivier Peslier to gain a neck's advantage with 200 yards to the wire. The move, though, proved a bit premature and gave Duke of Marmalade a chance to answer, which he did, putting his head in front in the final 80 yards en route to a half-length victory.
"I quickened up very well and was surprised to see another horse come with me," Murtagh told Racing Post. "But my horse has got the qualities of being a great horse. He's a great battler and has a great will to win."
It was nine lengths back to the third-place finisher, Youmzain. Red Rock Canyon, who set a strong pace until passed by Duke of Marmalade, wound up fourth. Ask, Petara Bay, Lucarno and Macarthur followed.
The final time was a fast 2:27.9 on a course rated good to firm.
O'Brien, who claimed his 14th group I win of the season, described Duke of Marmalade as "a rare racehorse."
The trainer won the King George last year with Dylan Thomas.
"This is marvelous and I'm delighted for everybody," O'Brien told Racing Post. "When he goes to the front he's lazy and Johnny gave him a marvelous ride. Johnny hadn't planned on taking it up as early as he did, so he was ready for another horse to come to him."
The victory knocked down Duke of Marmalade's price for the Prix de l'Arc de Triomphe (Fr-I), but O'Brien said the likely plan is to return the Irish-bred bay to the 1 1/4-mile distance with races such as the Juddmonte (Eng-I) and possibly the Irish Champion Stakes (Ire-I) in mind.