Mansony produced a blistering finish to beat Justified and Steel Band in the Kerrygold Champion Chase on the opening day of the Punchestown Festival.
Arthur Moore's charge came from the back of the field to win a race which had been led for a long time by last year's champion two-miler Newmill.
Tony McCoy had looked set for victory on Justified until Davy Russell brought the French-bred winner home.
Newmill, the 2006 winner and 13-8 favourite, finished in fifth place.
"He has always been a smashing horse and I hbave been looking forward to putting him in this company," said winning trainer Moore.
"I was a bit frightened he could have been done for speed early on, but Davy gave him a great ride.
"He would have preferred softer ground but he has now gone and won in Grade One company and we will have to go down that route with him next year.
"He will also be fine over two and a half miles. He is entered later in the week, but it is unlikely he will run."
Kerrygold Champion Chase (Punchestown)
1. Mansony (D Russell) 13-2
2. Justified (A P McCoy) 9-2
3. Steel Band (D J Casey) 40-1