A seven-try second-half onslaught, spearheaded by the pace and class of Saints' Anthony Sullivan, continued Leeds' nightmare start to the Super League season.
A magnificent hat-trick from the Great Britain winger was only part of Leeds problems as they crashed to their fourth defeat in a row.
Sullivan's first and third tries covered around 75 yards each and even his middle effort, from Chris Joynt's break, was a classic example of the wing-man's art.
"It's the first time I've coached a player on genuine speed," said his coach, Ian Millward, who has still not lost with Saints. "I've told him we're going to have some fun this season."
Sean Long was another major contributor for Saints, scoring a try and setting one up for Kevin Iro at the start of the second half, to put the visitors' noses in front. Chris Smith and Apollo Perelini added the other second-half tries before Ryan Sheridan scored one that did nothing to disguise Leeds' plight.
"That capitulation and poor one-on-one defence was very, very disappointing," said their coach, Dean Lance, who refused to blame the absence of leading players or the prospect of a Challenge Cup final at the end of the month for the substandard performance.
The wonder of the first half-hour was that Saints, with all the possession they could have asked for, failed to add to their early try from Tommy Martyn when he latched on to Long's little kick.
Leeds' defence already looked lethargic but Saints failed to take advantage and led by only 8-2, despite their domination, when the home side mounted their first serious attack.
Sheridan sent Kevin Sinfield through a gap and David Barnhill was in support for the converted try that tied the scores.
For all the battering they had taken, Leeds could have been in front at the break, had Francis Cummins and Barnhill not had tries disallowed.
"Maybe that would have made a difference," Lance said. In fact, Leeds were so poor in a second half that recalled the bad old days before they became the formidable proposition they have been of late, that it would have taken a lot more than a couple of tries to alter the course of what became a romp for St Helens.
Much of the stunned Headingley crowd headed for the exits long before the end while the South Stand faithful, in reference to Lance's predecessor, chanted: "There's only one Graham Murray."
Leeds: Cummings; Walker, Blackmore, Senior, St Hilaire; Powell, Sheridan; Fleary, Jackson, Ward, Hay, Barnhill, Sinfield. Substitutes used: Farrell, Jones-Buchanan, Rivett, Lawford.
St Helens: Wellens; Smith, Iro, Hoppe, Sullivan; Martyn, Long; Perelini, Cunningham, O'Neill, Joynt, Tuilagi, Sculforthe. Substitutes used: Henare, Matautia, Stankevitch. Substitute not used: Hall.
Referee: S Cummings (Widnes).Reuse content