Sir Percy, trained by Marcus Tregoning and ridden by Martin Dwyer, won the Vodafone Derby, snatching victory from Dragon Dancer in a dramatic finish.
Dwyer celebrates a dramatic win aboard Sir Percy
The 6-1 shot crept up on the rail to beat the 66-1 chance by a short head, with Aidan O'Brien's Dylan Thomas (25-1) in third, a further head back.
Hala Bek was fourth and pre-race favourite Visindar came home fifth.
Kieren Fallon's mount Horatio Nelson suffered a fracture to his front left leg and had to be put down.
Sir Percy was towards the rear of the field towards Tattenham Corner as Dragon Dancer and Dylan Thomas disputed the lead.
But Hala Bek and Sir Percy suddenly caught up with the field, although a jink to the right cost Hala Bek vital ground.
Visindar never really lived up to his billing as 2-1 favourite, and pre-race worries about Horatio Nelson, which were started by a look of concern on the face of jockey Fallon, proved to be accurate.
Afterwards, Dwyer was keen to pay tribute to the team effort that helped pull off one of the closest Derby wins in years.
The injuries were considered to be too severe to be repaired and the horse has therefore been euthanized
Horseracing Regulatory Authority statement on Horatio Nelson, who had to be put down
"It all opened up for me but it's all down to one man, Marcus and his team," he told BBC Sport.
"It was a rough race and I had to go where I could to get a run. He pulled a muscle and was very sore after the Guineas. He missed a fair bit of work and I didn't see him until last week.
"He showed a tremendous turn of foot there. I'm so pleased - it's fantastic."
Owner Tregoning was also keen to share the glory.
"Everyone has done a great job with him at home (to get him back after the Guineas) - my vet, physiotherapist and all the staff," he said.
"I have to thank Sheikh Hamdan for this as he has allowed me to train the horse on his gallops and he has been really interested in all the work he has done."
But the injury to Horatio Nelson cast a cloud over the afternoon's events.
Concerns about the horse's soundness had emerged down at the start when Fallon began trotting the colt around under the watchful eye of trainer Aidan O'Brien.
After consultation with a racecourse vet Horatio Nelson was allowed to take his chance. He raced in touch with the leaders and was in seventh place when the horse stumbled and was immediately pulled up by Fallon.
It later transpired the colt's injuries were too serious and he was put down. A Horseracing Regulatory Authority statement said: "Following the race the horse was taken to the veterinary treatment centre and several X-rays were taken of the injured leg.
"They revealed that Horatio Nelson had suffered fractures of the cannon and sesamoid bones and a dislocation of the fetlock joint.
"Sadly the injuries were considered to be too severe to be repaired and the horse has therefore been euthanized."
Vodafone Derby result (Espom):
1 Sir Percy Martin Dwyer 6-1
2 Dragon Dancer D Holland 66-1
3 Dylan Thomas J Murtagh 25-1
18 ran. Dist short head, head