The Coral-Eclipse Stakes hero had impressed in the Dante Stakes on his previous visit to the Knavesmire in May and improved on that with an authoritative display.
Powering past Benbatl, who had hit the front two furlongs from home, Roaring Lion (3-1) quickly went clear under Oisin Murphy to give the jockey and trainer John Gosden their first triumph in the mile-and-a-quarter showpiece.
King George victor Poet’s Word, the 8-5 favourite, stuck on well for second place, after being short of room at a crucial stage in the straight, but was still three and a quarter lengths behind the convincing winner.
Thundering Blue came home strongly to claim third and vindicate the decision of owner Clive Washbourn and trainer David Menuisier to supplement the 50-1 chance at a cost of £75,000, earning £114,000 for their enterprise.
Saxon Warrior was fourth for Aidan O’Brien.
Gosden said: “He’s run a lovely race. They went an honest pace. You’d got Dubai World Cup winners in there, Poet’s Word came at him and ran a blinder. There were no hiding places out there.
“We were delighted with him - I was expecting a big performance from him. This horse ran a great race in the Derby. He won his Eclipse well and he’s got better through the season. He’s got bigger and stronger.
“He had a difficult spring, but he’s done nothing but improve since.”