Fallon had an anxious wait before clinching victory at Longchamp
Kieren Fallon had to survive a lengthy stewards' inquiry before Dylan Thomas gave Aidan O'Brien his first Prix de l'Arc de Triomphe victory at Longchamp.
Fallon held off the challenge of Richard Hughes's mount Youmzain in a thrilling finish.
However, Dylan Thomas interfered with both Zambezi Sun and Soldier Of Fortune while drifting across to the rail in the closing stages.
But after a 30-minute wait the original placings were confirmed.
Frankie Dettori's Epzom Derby winner Authorized, who went off as the evens favourite, was never in contention and finished well down the field.
Soldier Of Fortune hit the front around three furlongs from home but could never get away from his rivals, while Zambezi Sun never managed to quite get on terms with the leaders.
Dylan Thomas, the dual Irish Champion Stakes winner, hit the front inside the final furlong under Fallon but Youmzain, trained by Mick Channon, threw down a big challenge.
But Fallon managed to get one more effort from the son of Danehill to claim victory.
It was a boost for the Irishman, who is currently banned from racing in Britain and appears at the Old Bailey on Monday to face charges relating to allegedly defrauding customers of Betfair.
After the stewards confirmed the victory a relieved Fallon said: "That was the longest 30 minutes of my life. I was very worried but the boys were confident I wouldn't lose it.
"It is one of the best days of my life, he is one of my favourite horses."
O'Brien told BBC Sport: "He has hardly ever been beaten over a mile and a half. The ground is a big thing for him but he has overcome that today. He is a super horse.
"Kieren gave him a masterful ride and he is a master in the saddle.
"This is a marvellous race, we have always wanted to have the winner and great credit must go to everybody at home. There are so many people involved that have made this happen."
Prix de l'Arc de Triomphe
1. Dylan Thomas (K Fallon) 11-2
2. Youmzain (R Hughes) 66-1
3. Sagara (T Gillet) 33-1