Fascinating Rock triumphed ahead of Found and Jack Hobbs to win the Champion Stakes at Ascot.
Pat Smullen guided his mount, trained in Ireland by Dermot Weld, to the front in the final furlong to score by one and a quarter lengths.
The 10-1 winner relished the good to soft ground and Smullen said the four-year-old had been "underestimated".
Favourite Solow won the Queen Elizabeth II Stakes, while English and Irish Guineas winner Gleneagles was sixth.
Simple Verse, who was reinstated as St Leger winner after an appeal, triumphed again in the Fillies and Mares Stakes on British Champions Day.
Frankie Dettori took the opening Long Distance Cup on 6-1 chance Flying Officer for trainer John Gosden.
The Charlie Hills-trained favourite Muhaarar was a highly impressive victor under Paul Hanagan in the Sprint Stakes, powering two lengths clear of Twilight Son.
Winners from around the globe
In a corner of Berkshire, there was a real international feel to this meeting.
Two Italian jockeys, French and Irish combinations, a victory for Qatar and a Dubai-born trainer and owner were among the winners.
Irish trainer Weld, whose middle name could be 'Shrewd', seemed unsurprised by Fascinating Rock's victory despite his odds of 10-1.
"Fascinating Rock has always had the potential to do what he did today, and I had this race planned for about six months," said Weld.
Meanwhile, super sprinter Muhaarar, owned by Dubai's Sheikh Hamdan Al Maktoum, looks a serious prospect as a stallion when he begins a stud career.
Boy from Brazil is top jockey
Silvestre de Sousa was crowned the season's champion Flat jockey, watched by family and friends, including his mother Estelina who had flown over from Brazil.
"When I first came to Europe I just wanted to have one ride, to get a feel of the turf, and today - look where I am," said the rider, 34, whose mum called him "very beautiful".
His childhood friend Ismael da Costa, resplendent in a black bow tie, was among those on hand to witness him lifting the trophy.
"I'm very proud - it's a dream come true. We grew up together near Recife. He is shy, quiet, but a very funny guy," the 31-year-old said.
"There are so many good jockeys from Britain and Ireland, I don't believe anyone will do this again - a rider from outside Europe. He's put in a lot of hard work."
|Champions Day winners|
|Long Distance Cup - Flying Officer 6-1 (Trainer John Gosden, jockey Frankie Dettori)|
|Sprint Stakes - Muhaarar 5-2 fav (Charlie Hills, Paul Hanagan)|
|Fillies and Mares Stakes - Simple Verse 5-1 (Ralph Beckett/Andrea Atzeni)|
|Queen Elizabeth II Stakes - Solow 11-10 fav (Freddie Head/ Maxime Guyon)|
|Champion Stakes - Fascinating Rock 10-1 (Dermot Weld, Pat Smullen)|
|Balmoral Handicap - Mussadas 7-1 (Saeed bin Suroor, Edward Greatrex)|
'The best day's racing ever'
The fifth running of British Champions Day, which aims to provide a high-class finale to the Flat racing season, finally brought victory for one of the day's leading figures.
Sheikh Fahad Al Thani, whose family business Qipco are the headline sponsor, celebrated his first win at the meeting as an owner when Simple Verse scored.
Ridden by Andrea Atzeni in the colours of Qatar Racing, the Ralph Beckett-trained Simple Verse wore down Journey in the closing stages.
Sheikh Fahad said: "This feels like the best day's racing ever, and this race means a lot to us. Having a winner on Champions' Day is wonderful, and we've waited five years."
And finally ...
A crowd of nearly 31,000 gathered for the meeting, which is now established in the racing calendar despite some concerns that an autumn billing risks unfavourable weather and clashes with the global schedule.
That crowd includes hundreds of students who gained fee entry, and there is a noticeably fresher feel to the whole day.
Ascot, once considered the bastion of stuffy top and bowler-hatted types, had a 'tweet wall' and #ChampionsDay hashtags painted onto the parade ring lawn.
Perhaps fitting then that a 17-year-old jockey should win the final contest - Edward Greatrex guiding home Mussadas for Godolphin trainer Saeed bin Suroor.
After racing, Paloma Faith did a DJ set as punters danced on the grandstand concourse.
"Get Lucky" was one of the early tracks. And some looked as though they had.
|BBC Sport racing correspondent Cornelius Lysaght|
|"Champions Day may take a few more years to rival Arc weekend in Paris or the US's Breeders' Cup, but in front of 30,967 this feels more and more special.|
|"Champions-wise we saw two exceptional talents. Solow marches on, again doing all that was asked of him in the QEII and extending his unbeaten run. He will be back next year and could rack up a very long sequence.|
|"In the Sprint, Muhaarar demonstrated himself to be the best European speedster since his dad Oasis Dream in 2003; sadly stud duties now beckon."|