New Approach, ridden by Kevin Manning and trained by Jim Bolger, held off the challenge of Tartan Bearer to win the Epsom Derby at 5-1.
Last year's champion two-year-old, who was second in this year's English and Irish Guineas, won by half a length.
The horse's participation at Epsom was in doubt up until earlier this week but trainer Bolger opted to run him.
The favourite Casual Conquest finished back in third, just ahead of Doctor Fremantle.
Manning's mount pulled strongly early on and he had to work hard to get him settled but as the race hotted up, he made a crucial move from the outside to the inside rail after spotting a gap.
He seized the lead from Tartan Bearer, ridden by Ryan Moore, and kept his cool to take the lead and then hold off the challenge of his rival to the line.
Bolger had indicated in April the ante-post favourite of the time was an unlikely runner in the Classic.
As a result of the trainer's comments, nearly all bookmakers withdrew the colt from their markets and even a week before the race, he said that he had left the horse in the Classic "by mistake".
But he performed a dramatic U-turn at the declaration stage for the race last Monday and admitted the decision to run New Approach was worth it.
"Winning the Derby is very special. I thought before the race it would be up there with the best. I very willingly say now it is the best," he said.
"There is no need to emphasise the mistake. It was the biggest mistake I have made in a long time. But it was fortuitous and it has worked out for the best.
"I am just extremely lucky and thankful I did make the mistake. I admitted the mistake from the word go and I don't see why people should have had such a big problem. I put my hands up and admitted the mistake.
"But full credit to Kevin Manning. He had a big job to do but he got it done."
Manning said: "He has got a serious, serious engine. He has always been crying out for this trip.
"He took a bit of a tug early on and got further back than I would have liked, but coming down the hill I had loads of horse and just hoped the gaps would come.
606: DEBATE
"He's just very, very classy. He gets a mile and a half but he is very pacy. To do what he has done he has to be very pacy.
"The ground in the Irish Guineas was just way too firm for him. To win this is every jockey's dream."
Epsom Derby
1. New Approach (K J Manning) 5-1
2. Tartan Bearer (R L Moore) 6-1
3. Casual Conquest (P J Smullen) 7-2 Fav
16 ran. Dist: ½, 4½
Also:
4. Doctor Fremantle 5. Washington Irving 6. Alessandro Volta
7. Rio de la Plata 8. Tajaaweed 9. Bouguereau 10. Curtain Call
11. Frozen Fire 12. King of Rome 13. Alan Devonshire
14. Kandahar Run 15. Bashkirov 16. Maidstone Mix
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