The South Yorkshire club led 24-16 heading towards the hour mark at Halifax on Saturday and, despite the dismissal of enraged prop Liam Higgins following an ugly brawl, looked well set to go on and win in their first appearance at the showpiece event.
However, the absorbing contest suddenly turned in a pivotal 60 seconds and Mark Aston’s side endured a debilitating collapse, conceding five tries in the final quarter as rampant Leigh hoisted the cup for the fourth time.
The crucial point came when Sheffield’s threatening full-back, Quentin Laulu-Togagae, strangely ignored a clear three-man overlap which would certainly have profited Armstrong with a try.
Instead, he was tackled near their opponents’ goalline before Matt Gardner produced a 70m intercept to turn defence into attack, setting up position for on-loan Wigan prop Tom Spencer to score and reignite Leigh hopes.
When Laulu-Togagae then spilled the excellent Martin Ridyard’s high kick under no pressure, they invited Leigh in again with Stuart Littler eventually scrambling over in the 62nd minute for a 26-24 advantage they would never relinquish.
It was bitterly disappointing for Sheffield, who had also seen their Challenge Cup dreams evaporate a week earlier against Super League strugglers London Broncos and must now quickly recover to concentrate on the defence of their Championship title.
They had initially seemed to cope best when both sides were reduced to 12 men in the 48th minute.
Ex-Hull FC front-row Higgins, who turned 30 the day before the game, should really have known better than reacting when being caught by the elbow of Gregg McNally as the Leigh full-back tried to wriggle him off to play the ball.
It is always difficult to prove intent in those instances; admittedly, McNally could not see where his arm was lashing with tackler Higgins on top although he runs the risk of contact by struggling in such a manner.
Regardless, the Sheffield prop took offence and rained down punches on his prone opponent, sparking a huge melee which also resulted in Leigh winger Steve Maden being red-carded for doing likewise to Higgins.
Sheffield were awarded the penalty for the initial offence, duly dispatched by Dominic Brambani to put them 18-16 ahead and then, after Laulu-Togagae crucially denied Gardner a try, the Samoan’s long pass sent in winger Scott Turner for a strong finish which was once more improved.
Then came that glorious chance to take complete control.
“I was there ready and their heads would have gone then,” Armstrong told the Yorkshire Post.
“We were comfortable and I felt we could have pushed on and won the game.
“We missed a couple of opportunities including that one but I really think that incident (brawl) changed the game.
“You’ve just got to keep your head in finals. You’ve got to keep your head cool haven’t you as there’s only one shot isn’t there?
“Obviously, if you’re swinging like that it probably was a red but it’s the heat of the moment.
“I think there were a couple of reds, to be fair, as there was so many punches going in but maybe it all could have been put on report as it was a final.”
By the time Armstrong got his try in the final seconds, the game was already over with Leigh having profited with further tries from Sean Penkywicz – who led Halifax to success last season – plus Ryan Brierley and Tommy Goulden.
Brierley also dropped a goal and Ridyard, whose kicking tormented Sheffield throughout, finished with seven goals.
Sheffield, second in the Championship, had won at Leigh in the play-offs in each of the last three seasons but their dominance over them was finally broken here.
Armstrong, who scored the winning try for Leigh when they defeated Halifax in the 2011 Northern Rail Cup final, added: “You can’t dwell on it. You’ve just got to take it on the chin. The league’s all we’ve got left now so we’ve just got to play for that.
“We got to the (NRC) semis last year and final this time so it is progression. We’ll aim to win it next year but we should have won this one; we were in control.”
The lead had changed four times with Matt Garside, Menzie Yere and Misi Taulapapa scoring first-half tries for Sheffield.
It was perhaps a sign of their ability to self-implode, however, that they messed up after Taulapapa’s 37th-minute effort to gift Leigh the chance to score on the hooter, Anthony Bate adding to Littler’s first score to narrow Eagles’ interval lead to just 16-14.
Leigh Centurions: McNally; Maden, Littler, Gardner, Pownall; Ridyard, Brierley; Ostick, Beswick, Spencer, Finnigan, Goulden, Hopkins. Substitutes: Penkywicz, Thornley, Bate, Parker.
Sheffield Eagles: Laulu-Togagae; Taulapapa, Armstrong, Yere, Turner; Walker, Brambani; Battye, Henderson, Stringer, Garside, Straugheir, Hirst. Substitutes: Davey, Higgins, Knowles, Green.
Referee: C Leatherbarrow (RFL).