The Red Rose youngsters suffered a 42-26 defeat to Ireland Under-20s, who scored 21 points in the final 20 minutes to win it.
Sleightholme had helped England to take control, scoring his side's first try of the tournament and adding a second after a fine assist from Saints team-mate Fraser Dingwall.
Dingwall was skippering England from the centres, with another Northampton player, lock Alex Coles, also starting the game held in Santa Fe, Argentina.
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England had taken an early lead, with Manu Vunipola sending over a penalty inside five minutes, before Sleightholme extended the lead with a try in the corner, taking advantage of an overlap on the right wing.
Jake Flannery jinked his way over under the posts from close range after a period of Ireland pressure, converting his own score.
Vunipola sent over a second penalty for a six-point lead but Ireland responded immediately with Stewart Moore dotting down and Flannery converting for 14-13.
England lost Aaron Hinkley for 10 minutes for a dangerous tackle but they held out during that period and soon had the lead with a third penalty for Vunipola.
A second try came soon after with Sleightholme collecting Dingwall’s crossfield kick to dive over in the corner and a 21-14 lead.
However, momentum swung back Ireland's way with Vunipola yellow carded for a high tackle and from the resulting lineout and drive David McCann bundled over with the conversion levelling the score.
Tom Seabrook flew over on the left wing for a third England try, courtesy of a superb offload from Hinkley to release his Gloucester team-mate.
McCann had a second try and a fourth for Ireland as they sealed the bonus point with Ben Healy's conversion also giving them a 28-26 lead.
Alfie Barbeary was red carded for an illegal tackle and soon after Ireland had a fifth try through Ben Healy which he also converted.
John Hodnett's late score added further gloss to the score.
England will next take on Italy on Saturday (KO 7.30pm BST) in Sante Fe before ending their Pool B campaign against Australia on June 12.