Wales sealed a historic 3-1 victory to reach the semi-finals of a major tournament for the first time in their history, Allen playing the full 90 minutes and producing another excellent performance in the middle.
Allen helps Wales to record-breaking win in Lille
The 26-year-old was however fortunate that his teammates produced a stirring comeback after he had been at fault for Belgium's opening goal, giving possession away with a misplaced pass that led to Radja Nainggolan's sublime strike from distance.
But skipper Ashley Williams headed Chris Coleman's men level before a stunning goal from Hal Robson-Kanu, selling his man with a fantastic turn before firing into the bottom corner, put them ahead in the second-half.
Sam Vokes headed home a third late on to ensure jubilant celebrations at the expense of Reds trio Benteke, Mignolet and Origi - none of whom came off the bench to feature for Belgium.
Speaking afterwards, Allen admitted that he "never thought" Wales would be "in this position" of reaching the final four, adding: "What an achievement for everyone in the country and everyone involved. This is a moment that means so much to everyone."
Allen even warned that it will take "something special" to stop the Dragons, who have won all but one of their games at the tournament besides a 2-1 loss to England, inflicted by Reds teammate Daniel Sturridge.
Benteke's future in the frame after Belgium exit
Danny Ward was an unused substitute, having not played any minutes since making his first start for Wales in their opening Group B win over Slovakia.
They will meet Cristiano Ronaldo's Portugal, who are yet to win a game inside 90 minutes so far this summer, in Lyon on Wednesday night.
But Wales will be unable to call upon Aaron Ramsey and Ben Davies, who have been integral to their success to this point, meaning Allen could be even more important to the side than he has been so far.
His performances have been so impressive, Liverpool have reportedly upped their asking price for the midfielder to £14 million - insisting they will not let him go cheaply this summer, if he is to leave, though he has just one year remaining on his contract.
Belgium's exit, meanwhile, frees up Benteke to discuss his future. Crystal Palace had a £25 million bid for the out-of-favour striker rejected, with Liverpool wanting closer to £30 million, and the 25-year-old wants a wage increase to move to Palace, according to reports.
Meanwhile, Germany will face either France or Iceland in Marseille on Thursday after they required a penalty shoot-out to edge past a resilient Italy side.
Can waiting for first Euros minutes with Germany
Emre Can was an unused substitute for the fourth successive game as Joachim Low's men drew 1-1 after extra time. Arsenal playmaker Mesut Ozil's strike was cancelled out by Leonardo Bonucci's penalty with 12 minutes remaining.
Neither could be split after extra-time, forcing the game to go to penalties where the Germans won 6-5 after a dramatic shootout, missing more spot-kicks (three) within the space of a few minutes than in their entire shootout history.
It is unlikely Can will make his first appearance in the semis, but injury to defensive midfielder Sami Khedira and centre-back Mats Hummels' suspension at least boosts him up the pecking order a little.
Having yet to play for the national team anywhere but at right-back, where Bayern Munich's Joshua Kimmich has played throughout the tournament, Can could be called upon to step into defence - although it's likely he will remain one of their first-choice reserves.
With Jürgen Klopp's first-team having returned to pre-season training on Saturday, it's unlikely that Ward, Allen and Can will join up with their teammates until the tour of the United States in mid-to-late July.
Those already eliminated from the Euros, dependent on the stage they were knocked out, will have at least two more weeks' holidays before being required to return to Melwood.