England winger Ryan Hall scored two tries at Langtree Park, while Kylie Leuluai also crossed, as the Super League champions atoned for last year's 46-6 hammering to leave Saints still looking for a first home win under coach Nathan Brown.
The bright spot for St Helens was an impressive debut by 18-year-old England academy international centre Mark Percival, who carved out two consolation tries for his winger Francis Meli, while Jonny Lomax crossed early on.
The match began badly for Saints when Jordan Turner put the ball out on the full from the kick-off and Leeds captain Kevin Sinfield gave his side the lead with a third-minute penalty.
St Helens were the first to demonstrate their attacking prowess, however, and full-back Lomax followed up last week's hat-trick against Bradford by scoring the game's first try, taking Jon Wilkin's pass and beating two defenders to touch down.
Gary Wheeler was unable to add the conversion and Sinfield drew his side level with a second penalty before making the break for Leeds' opening try. He dummied his way through a gap in the first line of defence and Jamie Jones-Buchanan and Danny McGuire continued the move, enabling Hall to finish at the corner for his first try.
Sinfield's failure to add the conversion ended his goalkicking run going back to September, but Leeds just about deserved their 8-4 interval lead.
Saints always threatened through the clever offloading of Sia Soliola and Tony Puletua but it was young Percival who brought the crowd to life with a tremendous break out of his own half.
Leeds centre Joel Moon then demonstrated his class to carve out an opening before prop Kylie Leuluai went over for the visitors' second try. The ever-dangerous Hall was the architect, collecting McGuire's high kick and defender Louie McCarthy-Scarsbrook palmed his pass into the grateful arms of Leuluai, who forced his way over from five metres out.
Saints suffered more misery when Moon produced another touch of class to set up a limping Hall for his second try. Sinfield took his goal tally to four from five attempts to make it 20-4 but Saints finished with a flourish, with Percival capping a memorable start to his Super League career with long-range breaks which were finished off by Meli.
In the night's other game, London Broncos climbed off the bottom of the table with a seven-try romp at fellow strugglers Salford Reds as they secured a 38-4 victory.
The Broncos had conceded 138 points in four opening defeats but were in front 22-0 at half-time, Kieran Dixon twice going over while Michael Channing and Luke Dorn also crossed.
They continued to dominate in the second half courtesy of a pair of tries from Chris Bailey and another score from Luke Colborn, with Wayne Godwin scoring Salford's only points.
Salford had won their last three home matches against London since 2009 but the Broncos would have taken the lead had Michael Witt's kick bounced more kindly for Dixon. But Dixon was not to be denied after 16 minutes when Jack Murphy and Danny Williams missed a Craig Gower kick, with Witt tagging on the conversion.
Mark Sneyd's midfield break caused London problems but his kick was too deep for Williams, then Marc Sneyd had a try disallowed before Dan Sarginson almost wriggled over for the Broncos.
London stretched the lead to 10-0 on the half hour after Ashley Gibson failed to collect a Craig Gower kick and Channing accepted the gift.
Salford's plight worsened just before the break when Dixon followed his own kick ahead to touch down and Witt's conversion made it 16-0. Dorn added to Salford's woes by going over for a converted try to make it 22-0 at the break.
Dorn went close early in the second half after another Gower kick rebounded off the post.
Sneyd's weaving run almost brought Salford's first try but London were doing the basics right. Bailey added a fifth try for London 10 minutes from time before Godwin's late consolation for Salford.
The home side's misery was compounded with Bailey grabbing his second try and Colbon going over in the last minute for London.