Subjectivist provided Mark Johnston with a fourth Gold Cup at Royal Ascot, as Stradivarius - chasing a fourth win of his own - met trouble in running and could only finish fourth.
Joe Fanning sat second throughout as Amhran Na Bhfiann set steady fractions at the head of affairs, while Frankie Dettori on Stradivarius was happy to let the race unfold in front of him.
Fanning kicked clear with over two furlongs to run, just before the field turned into the straight, and as the runners bunched up Dettori found a wall of horses blocking his path on the favourite.
Subjectivist - so impressive in the Dubai Gold Cup last time out and a Group One winner in France before that - took three lengths out of the field and was never likely to be found out for a lack of stamina.
As he kicked clear Amhran Na Bhfiann was weakening rapidly - right into Dettori’s path - and all of a sudden Stradivarius was up against it.
Tony Mullins’ mare Princess Zoe came out of the pack to chase Subjectivist and although Dettori tried valiantly to weave his way through, he found himself with too much to do.
Subjectivist (13-2) had five lengths to spare over Princess Zoe, with Spanish Mission a further half-length back in third, just ahead of the 5-6 market leader.
Hearn, credited with revolutionising darts and snooker and heavily involved in the boxing industry as founder of Matchroom Sport, was present on course with his wife, Susan - the brainchild behind Mascalls Stud which bred the Johnston-trained winner.
He tweeted: “Sometimes dreams do come true!! Subjectivist wins the Ascot Gold cup. Without doubt one of the greatest days of my life. So proud of my wife Susan who bred the horse at her Mascalls Stud. Thanks to Mark Johnston and all the connections - an unforgettable day.”
Speaking to ITV Racing, Hearn said: “It’s taken 40 years to be an overnight success!
“The amount of work that goes on behind the scenes that people don’t see, staying up late at night, getting up early in the morning, having a disaster here and there but sticking to the job, saying you love the business and love the horses I breed.
“She has the mare, Reckoning, who throws up winner after winner, Sir Ron (Priestley) runs on Saturday (in the Hardwicke), Alba Rose is coming out soon, there’s a two-year-old with Mark Johnston - I’m very proud of her.
“It’s a very tough game, but days like today make it all worthwhile.”
Susan Hearn said: “It’s normally known as Barry Hearn’s stud, but it is actually me who does all the work! This is unbelievable and I still can’t believe it.”
Additional reporting by PA.