The Bravehearts lived up to their nickname on a thrilling night in Cumbria, finding a way to win after squandering a 20-4 lead.
New Warrington signing Matty Russell's try, captain Danny Brough's conversion and then Daniel Foster's knock-on with a second to go ensured a Scottish success story that will live long in the memory, perhaps even bettering their 2008 win over Fiji.
McCormack and his coaches could barely watch as the video referee reviewed Foster's claim of a try - the sixth time he was called into action on the night - with the outpouring of emotion that followed one that had to be seen to be believed.
The fathers of the Australian players in the squad belted out 'Flower of Scotland' while wearing kilts, with the anthem later shaking Workington's stand as Scotland celebrated a win which, if they beat Italy at the same venue on Sunday, could take them to the quarter-finals.
For McCormack, a school teacher on holiday, it was a deserved success, however it came.
"Many people didn't give us a chance looking at that Tonga team, who showed glimpses, but we found a way to win. That is up there with the best Scotland have ever had," he said.
"There are certain decisions you can argue with, but some go your way and you can't look at that and say we were lucky. We earned everything we got. Tonga were brilliant for 30 minutes, but we found a way to win.
"We spoke before the game about defending our goal-line sets, restricting offloads and taking care of their playmakers. In the second half they played like they can, but in the last 10 we held firm. My heart was in my mouth and I was not sitting comfortably, but we found a way to win. I am very proud to be associated with that."
Man of Steel Danny Brough was McCormack's eyes and ears on the field, with the captain having to make himself heard in front of a biggest Workington crowd in 19 years of 7,630.
"I told the lads not to panic. They (Tonga) were scoring off missed tackles and I had to calm the boys down and tell them to earn the right to have the ball," said Brough, who can look back on a conversion which bounced over off the bar as being crucial.
"We were a little hit and miss at times and it wasn't pretty but World Cup games rarely are.
"You have the best players in the world going at it and it was really important for us to get that win to set the group up."
With pool fixtures against surprise package Italy (at Workington on Sunday) and the United States to come, Scotland have a real chance of progression.
"The atmosphere in West Cumbria has been brilliant. Let's hope that it can remain that way for the Italy game. We have to improve for Italy but we will," McCormack added.
For Tonga, it was a case of snatching victory from the jaws of defeat and then coughing it out again.
They thought they had won it when Foster celebrated with a second on the clock but no try was given, with the half-back judged to have knocked on, despite some suggestions Daniel Addy and Brough had stripped him of the ball.
"That's really gut-wrenching. We probably let ourselves down a bit," said captain Brent Kite.
"There were a few 50-50s and we didn't get them. We can't worry about that. We let ourselves down in the first half."
Coach Charlie Tonga suggested he was more concerned than his skipper about Foster's no-try, but shared the concerns about The Mate Ma'a error count.
"This is a bit of a wake-up call. We now have to regroup and ask the boys to go out and do their best for the rest of the tournament," he said.
"It was mentioned at a coaches meeting that a two-man strip would be a penalty and maybe that is something to look at.
"But besides that, we can't make excuses for our errors because we started really poorly."
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- Danny Brough