Desert Hero was easy to back at 7/1 but the ease in the betting proved no issue for William Haggas' runner, who always looked to be travelling smartly in behind horses.
The King George V Stakes winner could be spotted going well behind rivals, but Tom Marquand was found continually looking for racing room.
Gaps failed to appear as the field began to quicken but jockey Marquand spied a route down towards the inside rail and once in the clear, the royal runner stuck on strongly to deny Simon and Ed Crisford a quickfire double, with the front-running Chesspiece going down by a neck back in second.
The Aidan O'Brien-trained Espionage attempted to make his challenge towards the middle of the track under Ryan Moore but failed to pick up and was eventually eased off to come home trailing the field.
Tom Marquand, who was onboard Desert Hero when the pair scored at Royal Ascot, told ITV Racing: "Today was a testament to how tough he really is. While he had to be brave to find gaps at Royal Ascot, he had to be brave again today on ground that is extremely tough work.
"He's obviously improving, but it's been a fantastic job by the team to get him on this path - I'm thrilled. He filled me with confidence because of what he did at Royal Ascot, and while it was always going to be tougher on this ground, he's got a kick over a mile and a half. He was really tough, and you can't ask for more than that."
Of Desert Hero's St Leger chance, Marquand said: "He stays well and has shown he handles ground that will be no worse at the back-end of the season. Whether that's on the agenda I'll leave it to William [Haggas, trainer], Mr [John] Warren and His Majesty [joint-owner] to figure out. He's on the right path.
"We would all want to see it [a Royal runner in the St Leger] and how lucky are we that they are so involved in our sport, and we should celebrate every bit of success they have."
Maureen Haggas, who was representing the winning trainer, said: "It wasn't the easiest watch, but Tom said he always thought he was going to win. The important thing with this horse is getting him switched off early, which he did really well, and I think if you can do that you can let the rest of the race unfold. He wants to win and that counts for a lot.
"The obvious aim would be the St Leger, I suppose, and I think, with all these things, you never really know until you run in the race. We thought Storm The Stars would stay all day and he didn't quite get home in the St Leger, so you never really know until you try, but there's no reason not to try, is there?
"He's a little bit of a worrier, but he is getting better. When he arrived up here, he was sweating, and we washed him and he walked round, and the longer he was here, the more he was relaxing. Luke Carson rides him at home and has done a brilliant job, because he hasn't been the easiest horse and he's improved a hell of a lot from two to three, and he's still improving physically and mentally."
Paddy Power cut the winner from 16/1 to 6/1 to land the last Classic of the season at Doncaster, while the runner-up, Chesspiece, was also cut from 16/1 to 8/1.
