Lightning Spear benefitted from a patient ride by jockey Oisin Murphy to win the Sussex Stakes at Goodwood.
The 9-1 chance finished third last year but this time, once Murphy found a small gap late on, he went through it to win by a length and a half.
Expert Eye (4-1) was second for James Doyle and trainer Sir Michael Stoute, with Lord Glitters (10-1) third.
"What a horse, what a training performance. At seven years of age, it's outstanding," said Murphy.
The David Simcock-trained winner had come close to Group One success on a number of occasions, including losing by a short head to Rhododendron in May's Lockinge Stakes at Newbury.
But his Sussex Stakes victory was a first Group One success at the 16th attempt.
"He's deserved to win a major Group One," Murphy said. "He loves travelling behind heels and I was sure Expert Eye was going to fade away at some stage."
Much of the pre-race talk had been about the previously unbeaten Without Parole and although the son of Frankel was well placed early on in a slow-paced race, he faded out of contention to finish seventh.
BBC horse racing correspondent Cornelius Lysaght
Whatever the sport, followers love longevity, and in horse racing, opportunities to follow equine heroes for years on end tend to be confined to jumping, where stud value is less relevant because horses have generally been castrated.
But Lightning Spear is the exception that proves the rule.
Aged seven, and 'entire', he's been competing since August 2013, and has been knocking on the door of Group Ones for ages, going close but not winning.
Ironically, this will probably signal his career coming to the end - now he has that top-level success to his name he will be even more attractive for a stud career.