Richard Fahey's Mayson led his rivals a merry dance in the Darley July Cup at Newmarket.
The 20-1 chance was in front from an early stage in the hands of champion jockey Paul Hanagan and kicked clear a couple of furlongs out.
He never looked likely to relinquish his advantage, readily beating The Cheka by five lengths, with 7-2 favourite Society Rock third.
It was just a second Group One winner for both Fahey and Hanagan and their first in Britain.
Mayson had started off the season in fantastic form, with a Doncaster third followed by two victories on the Rowley Mile.
He disappointed subsequently at York and Newcastle, but bounced back to form in some style on his return to Headquarters.
The four-year-old appeared to be relishing the rain-softened ground out in front and it was clear he had a number of his opponents in trouble a long way out.
The Cheka and Society Rock were both doing their best to make inroads at the death, but were unable to reel in the emphatic winner.
It was a special result for both trainer and jockey, with Hanagan leaving his post as Fahey's stable jockey at the start of the season to become retained rider to leading owner Hamdan Al Maktoum.
Hanagan said: "It's very good. This is my first Group One winner in England, I've had one in France, but this is very special."
Fahey said: "It's fantastic for me and Paul. They don't realise he's still my number one and we borrow him from time to time!
"We thought he was very good, but it went wrong at York when he got his leg stuck in the stalls and the ground at Newcastle was unraceable the other day and he just got bogged down.
"The ground was a lot worse than it is today. At Newcastle, you put you foot on the track and water was coming through, but this track has taken the rain fantastic.
"We thought he'd win the Ayr Gold Cup last year and why he didn't win, I don't know.
"Things just didn't go right, but he's paying his way now and it's grand."
Fahey continued: "I'm delighted for the owners. David (Armstrong) is very hearty at the sales, but to actually breed one is fantastic. He wants to stick to the breeding and leave the buying to someone else!
"For the last 100 yards, I felt comfortable. Paul said when he asked him to go he just took off.
"He's in the Nunthorpe, but he got very upset at York in the stalls, so we'll see.
"We'll enjoy today."