The pair were neck and neck as they leaped the final jump before Native River, who came in third a year ago, won a sprint to the finish and ultimately triumphed by four and a half lengths.
Might Bite, bidding to give Nicky Henderson an unprecedented treble after the victories of Buveur D'Air in the Champion Hurdle and Altior in the Queen Mother Champion Chase, appeared to be going the better of the two as they turned for home.
However, this was only for a fleeting moment.
Dromore farmer Mackey had bred the horse from his own mare Native Mo, covered by Indian River, a son of the great Cadoudal.
"To be honest, I was just a passenger", said Johnson who won the race in 2000 with Looks Like Trouble. The filly gets 7lbs from the boys and that might just prove decisive although the concession was not quite enough to get Ms Parfois' head in front in the NH Chase on the opening day.
The victor will make £369,822 in prize money.
"Someone's got to finish second and we've made it a race".
Johnson had a willing partner, but the stewards hit him hard with £6,550 fine and seven-day ban for excessive use of the whip.More news: Ex-Trump campaign chair asks judge to dismiss charges
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"It's unreal", said the Venn Farm maestro.
"I knew Nico (de Boinville, on Might Bite) was there and to be fair to Might Bite, he's run a hell of race, but I think we just outstayed him".
"I think we got our head in front winging the last and then the ground on that track is the worst from the last to the line and we just tied up on it - pure and simple".
There was very little luck for the Irish with Jessica Harrington - last year's winning trainer - having to watch second-favourite Our Duke pull up while Road to Respect and Willie Mullins' Djakadam finished fifth.
With Sizing John out of the Gold Cup, it looks an extremely competitive renewal for the Timico Cheltenham Gold Cup.
And Elliott took the opening Triumph Hurdle with Farclas for his seventh victor of the week, and a fourth for 18-year-old jockey Jack Kennedy.