Three goals were netted inside the opening half-hour, with the young Magpies falling behind in the early stages of the encounter following Jake Evans' low drive and despite Sean Neave's close-range effort levelling the contest, Sammy Braybrooke's strike swiftly restored the Foxes' one-goal cushion.
Newcastle conceded a further three times in the space of six second-half minutes, with Evans completing his hat-trick with a quick-fire brace towards the hour mark, either side of Jayden Joseph's strike, and despite goals from Garang Kuol and Matheos Ferreira setting up a tense finale, Tommy Neale's stoppage-time strike confirmed maximum points for the Foxes.
Newcastle conceded in the sixth minute as Evans dangerously evaded Dylan Charlton on the right flank before managing to squeeze a low shot into the net via the front post.
Kuol, who represented Australia at the 2022 World Cup finals in Qatar, went close to equalising for the hosts as the Cairo-born youngster's volley from Leo Shahar's lofted cross to the back post was directed inches wide of the right-hand post.
In the 22nd minute, Newcastle found themselves back on level terms through Neave. After Scott Bailey's corner was fizzed into Jay Turner-Cooke on the edge of the box, the latter's drilled strike was deflected into the net by Neave, who was named on a first-team matchday bench for the first time during the Magpies' Emirates FA Cup fourth round win at Birmingham City last week, inside a crowded penalty area.
United's youngsters suffered a tough blow three minutes later, though, as Braybrooke restored Leicester's one-goal cushion as the central midfielder picked the ball up on the edge of the box before being allowed to lash a powerful drive beyond Magpies goalkeeper James Taylor.
Leicester doubled their advantage in the 54th minute as Evans grabbed his second goal of the night, latching onto Louis Page's cross from the byline before drilling the ball into the far right-hand corner.
The Foxes added a fourth in the 58th minute through Joseph as the Foxes full-back's strike from the edge of the box managed squirmed through the gloves of Taylor.
Evans completed his hat-trick on the hour mark as the City winger's strike from the edge of the box found its way into the bottom right-hand corner.
Kuol managed to pull a goal back for the young Magpies as the 20-year-old forward, who arrived at St. James' Park from Central Coast Mariners in January 2023, advanced into the box before calmly slotting the ball beyond Leicester goalkeeper Harry French.
Ferreira, who signed a full-time scholarship with the Magpies last summer, netted his first goal at under-21 level as the 16-year-old's winger intended cross floated into the left-hand corner.
Newcastle ought to have reduced the deficit further as the lively Ferreira teed up Alfie Harrison inside the box but the Keighley-born teenager's first-time effort was placed narrowly wide of the target while Kuol's low shot in the closing stages struck the base of the left-hand post.
United's hopes of a comeback ended in the third minute of stoppage-time as Neale netted Leicester's sixth goal of the night, drilling a powerful low effort into the bottom left-hand corner.
Newcastle United Under-21s: James Taylor, Leo Shahar (Rory Powell 46), Cathal Heffernan (c), Ciaran Thompson, Dylan Charlton, Scott Bailey (Joe Brayson 76), Jay Turner-Cooke, Alfie Harrison, Johnny Emerson (Matheos Ferreira 58), Garang Kuol, Sean Neave (Ellis Stanton 46).
Subs not used: Tyler Jones.
