20-1 shot Benefficient stayed on well to win the opening race at the Cheltenham Festival on Thursday.
Cooper told Channel 4: "It's unbelievable. I don't think it has quite sunk in yet. I have been dreaming of this since I was very small."
Dynaste, who was the pre-favourite, had looked on course to seal victory, but Benefficient went into the lead after the last fence to win by three lengths.