2003: three-year-old season

On his racecourse debut, Papineau started favourite for the Prix Aveu over 2400 metres at Saint-Cloud Racecourse on 2 April. Ridden by Christophe Soumillon he led for most of the way before being caught in the final strides and beaten a short head by Touch of Land, a colt who went on to win two editions of the Prix Dollar. At the end of the month over the same course and distance he recorded his first success, beating Well Dressed by one and a half lengths. In May, he was moved up in class for the Listed Prix de l'Avre at Longchamp Racecourse and started the 1/5 favourite against four opponents. Ridden by Soumillon, he raced in third place before taking the lead 100 metres out and won by a neck from Affirmative Action.^[5] On 1 June, Papineau started the 8/1 third favourite for the Group One Prix du Jockey Club at Chantilly Racecourse in which he was ridden by Frankie Dettori. After taking the lead in the straight he weakened in the closing stages and finished fifth of the seven runners behind Dalakhani. After the race, Dettori said "He was the first beaten and the ground was too fast for this colt".^[6]

At the end of the year, Papineau was transferred to Sheikh Mohammed's Godolphin Racing team and was trained from then on by Saeed bin Suroor. Before he raced again he was gelded.

2004: four-year-old season

Papineau made his British debut in a minor takes race over one and a half miles at Goodwood Racecourse on 19 May in which he was ridden by the Australian Kerrin McEvoy. Starting at 8/1 in a fourteen-runner field he overtook his stablemate Songlark inside the final furlong and went clear to win by two and a half lengths.^[7] Twelve days later, Papineau was stepped up in class and distance to contest the Group Three Henry II Stakes over two miles at Sandown Park Racecourse. He started the 9/4 second favourite against eight opponents including the Ascot Gold Cup winners Mr Dinos and Royal Rebel as well as the Sagaro Stakes winner Risk Seeker and Misternando, winner of ten races in 2003. Ridden by Dettori, he tracked the leaders before taking the lead approaching the final furlong, and won by one and a half lengths from Mr Dinos with New South Wales in third. Papineau was carrying 124 pounds, whilst Mr Dinos carried 131.^[8]

On 17 June, Papineau, again ridden by Dettori, started the 5/1 second favourite for the 196th running of the Gold Cup over two and a half miles at Ascot Racecourse. Mr Dinos (the 5/4 favourite), Royal Rebel, New South Wales and Misternando were again in opposition along with Brian Boru, Westerner, Ingrandire (Tenno Sho), Darasim (Prix Kergorlay) and Alcazar (Sagaro Stakes). After being restrained towards the rear of the field in the early stages, Papineau began to make progress in the last half mile. He overtook Westerner a furlong out and stayed on to defeat the French challenger by one and a half lengths, with Darasim taking third ahead of Royal Rebel.^[9] Dettori celebrated with a flying dismount and said This horse is top class – he can win a Group 1 over a mile and a half. I had faith in my horse's turn of foot, and if he could take me to the furlong marker, I knew I would outsprint anyone in the last furlong." The connections of the beaten horses were less charitable: Westerner's owner commented "the dope testing machine must be broken", whilst Mark Johnston, trainer of the third and fourth placed horses, objected to the use of ear plugs on the runner-up.^[10] The Godolphin team indicated that the horse would be brought back in distance to contest races such as the King George VI and Queen Elizabeth Stakes and the Prix de l'Arc de Triomphe^[11] but Papineau did not race again in 2004.

2005: five-year-old season

After an absence of almost eleven months, Papineau returned for the Yorkshire Cup on 13 May 2005. He started the 7/2 second favourite but finished fifth to Franklins Gardens after hanging to the right in the closing stages. The "Royal Ascot" meeting of 2005 took place at York Racecourse as Ascot Racecourse was being redeveloped. Papineau was made third favourite for the Gold Cup, but never looked likely to win and was virtually pulled-up by McEvoy in the straight and finished sixteenth of the seventeen runners behind Westerner.^[12]