A third defeat for St Helens in their last five games has undone the promising start made under new coach Justin Holbrook.
England internationals Kallum Watkins and Ryan Hall got the all-important tries in the last 16 minutes that enabled Leeds to come from 16-12 down, but the 2015 champions were hugely indebted to young guns Jack Walker and Jordan Lilley in helping them overcome the absence of seven senior players through injury and suspension.
Full-back Walker, just 17, produced an accomplished, man-of-the-match performance belying his tender years while Lilley, 21, proved a splendid replacement for suspended captain Danny McGuire on his first start of the season.
The victory lifted Leeds into second place in the table while St Helens were without leading scorer Mark Percival through suspension and unable to play new signing Ben Barba. They then suffered a body blow when scrum-half Matty Smith was forced off after only six minutes with a badly-cut eye after catching a stray arm from his own skipper Jon Wilkin.
The former England number seven had spent virtually his whole six minutes on defence as Leeds earned themselves three early penalties and forced two goal-line drop-outs without making the pressure tell.
Wilkin was forced to step into the role of chief playmaker but it was Smith’s replacement, centre Dominique Peyroux, who carved out the opening score, brushing off the attempted tackles of Joel Moon and Carl Ablett to get winger Adam Swift over for a 15th-minute try.
Tommy Makinson was unable to add the goal and Leeds drew level three minutes later when Moon evaded the challenge of Kyle Amor and stretched out of Peyroux’s tackle to score his 19th try of the season.
The impressive Moon then breached the Saints defence from deep inside his own half to create a clear-up scoring opportunity only for Liam Sutcliffe to send out a wayward final pass as he appeared to pull a hamstring.
Lilley, who struck the woodwork with his conversion attempt, made amends with a penalty four minutes before the break to edge his side into a 6-4 lead which would have been far different but for the defensive heroics of Walker. He pulled off a last-ditch tackle to deny centre Ryan Morgan in full flight for the line and then halted a rampaging James Roby with a crucial ankle tap.
The teenager, so cool under the high ball, then demonstrated his attacking threat with a break out of defence three minutes into the second half that created the position for Moon and Lilley to work second rower Stevie Ward over for Leeds’ second try.
Lilley’s second goal made it 12-4 but the game was blown wide open when Saints ran in two tries in a seven-minute spell.
Leeds substitute Ash Handley dropped the ball from Roby’s high kick and Peyroux got Makinson over the line before forward Louie McCarthy-Scarsbrook popped a sweet pass out of the tackle for the ever-alert Roby to touch down.
Makinson kicked both conversions to put his side four points in front but they were crucially reduced to 12 men on 62 minutes when Wilkin was sin-binned for a dangerous tackle on Mitch Garbutt.
And Wilkin could only watch in horror as Morgan fumbled Lilley’s high kick on the last tackle into the grateful arms of acting captain Watkins for the killer try, which Lilley goaled.
Moon then worked Hall over for Leeds’ fourth try to give them a cushion, which proved vital when prop Alex Walmsley forced his way over from close range for St Helens’ third try three minutes from the end.