The hosts battled back from a 16-point deficit to emerge victorious through second-half tries from Tonga'uiha, Jimmy Staples and Victor Worsnip, despite having finished the game with 14 men following a Luke Davidson red card.
Tonga’uiha had come off the bench at the Gardens, with Saints team-mates Oliver Scola, Aiden Ainsworth-Cave and James Pater all starting for England.
After an error-strewn start from both sides, it was Wales who registered the first points through a penalty awarded thanks to some overenthusiasm from the English pack.
Fly-half Carwyn Leggatt-Jones made no mistake from out in front to put the visitors ahead within three minutes.
Replacement tighthead prop Tonga’uiha was then introduced early on, with England’s Ollie Streeter having been injured in the build-up.
Torrential rain at the Gardens would go on to play a key part in the result, and indeed both teams struggled to find a foothold in the early exchanges.
All of the rugby was being played in England’s half, and Wales looked set to capitalise further thanks to some probing kicks from Leggatt-Jones.
The pressure eventually paid off, with inside centre Steffan Emanuel pouncing on a loose ball in the midfield and sliding in to score.
Leggatt-Jones converted and, thanks to two quick penalties, had nudged his side to a 16-0 lead at the break.
England started the second half with some much-needed attacking intent, Lucas Friday again the man to provide the impetus with a testing jab in behind the Welsh defence.
The Harlequins scrum-half was unable to gather, but England capitalised on the field position with their first score of the game, Tonga’uiha muscling over from short range at his club’s home ground.
England were suddenly galvanised and soon crossed again following a period of sustained pressure in the Welsh half.
Presented with a kickable penalty, skipper Connor Treacey opted for the corner and was rewarded for his faith in his side’s set piece as hooker Staples rumbled over on debut.
The conversion was missed, but England had deservedly clawed the deficit back to just five points.
The momentum was soon halted as Davidson was shown a red card for a high tackle.
But despite this setback, England held firm, and a valiant defensive display led to them being rewarded with a penalty deep in the Welsh half.
After the initial maul attempt was thwarted, Worsnip cut a superb line against the grain and crashed over to hand his side the lead with five minutes remaining.
The conversion was taken and before long England were knocking at the door for another, Saints forward Ainsworth-Cave coming close to sliding in out wide.
However, their lead was not to extend beyond three points, and Friday was happy to thump the ball into the stands to seal a remarkable comeback victory.
England now move on to face Scotland at Hive Stadium, Edinburgh in round two of the competition on Friday night (kick-off 7.15pm).
1. England Under-20s 19 Wales Under-20s 16
England Under-20s celebrated beating Wales Under-20s at the Gardens last Friday Photo: David Rogers
2. England Under-20s 19 Wales Under-20s 16
Saints forward Aiden Ainsworth-Cave Photo: David Rogers
