Victory ensured Harrington became the most successful woman trainer in Festival history and sees her join two-time victor Jenny Pitman and three-time victor Henrietta Knight in the Cheltenham Gold Cup Hall of Fame.
The 7-1 shot, who was tipped by City A.M.to prevail, capitalised on errors by the frontrunners on the second to last fence.
As many as nine horses were in with a winning chance on the home turn, out of the field of 13, but Sizing John (7/1) was able to use his two mile pace to quicken ahead at the last.
Tizzard is also well represented by NATIVE RIVER, beaten in the four-miler here last season yet much improved since.
"He's in great form and I've been telling anyone that has asked me that and I couldn't be happier with him", said Mullins.
At the other end of the career spectrum is his stablemate Native River.
"When I won the Grand National I was 25 and thought I was going to win everything - now I am 35 and realise I am not going to win everything so to win a Gold Cup is fantastic". He had the ideal prep when winning Newbury in February and 26% pf all stakes through the site have been on the proven stayer.
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Why is that? The facts of this case have been clear for a very long time at this point thanks to a very detailed DOJ report . He takes it, but gives it back just before leaving. "We want people to understand what happened, and they're going to".
"This is the jewel in the crown, absolutely awesome", said Harrington who joins two-time victor Jenny Pitman and triple winning trainer Henrietta Knight in the Gold Cup hall of fame. "This is the jewel in the crown--the race I've listened to and watched all my life", she continued. "He jumped like a buck and it's his jumping that got him there the whole way", she said.
The well-fancied Cue Card fell three out at the same fence as a year ago while Sizing John went to the front at the second last and followed that with a good jump at the last to add the Cheltenham Gold Cup to the Irish Gold Cup.
Owner Alan Potts names his horses after the mining term "sizing", which means to extract minerals by crushing.
"If you think about it, if Douvan hadn't been around, we'd have stayed at two miles".
Douvan (Fr) (Walk In The Park ), who has been found to have a stress fracture to his pelvis following a below-par run in Wednesday's G1 Queen Mother Champion Chase, has indeed been something of a nemesis for Sizing John, who has finished placed behind him on seven occasions, including being runner-up in last year's G1 Racing Post Arkle Trophy at the Festival when trained by Henry de Bromhead. "It is a first runner in the race for me".
Former Grand National winning jockey Robbie Power rode Sizing John and was quick to praise the Irish horse's trainer.
Harrington got off the mark at the meeting this week with Supasundae in the Coral Cup.