The 7-1 shot from Ireland beat Minella Rocco (18-1) and Native River (7-2) in the flagship race at the four-day meeting to complete a memorable double after landing the Irish Gold Cup last month.
"It's a dream come true. Our first runner in the race, the jockey's first ride in the race and trainer Jessica Harrington's first horse in the race," Sizing John's owner Alan Potts told the BBC. "It feels fantastic."
Lizzie Kelly, the first woman jockey in the race for 33 years, was unseated by her horse Tea for Two at the second fence.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
(Reporting by Ed Osmond; Editing by Alison Williams)
