Saints marked the arrival of their new coach, Daniel Anderson, with a convincing victory over the only club ahead of them in the Super League, narrowing the gap at the top to two points in the process.
Sean Long was the leading light with a near-perfect all-round kicking display and some incisive running, but this was a team performance to reassure the new man, if he ever had any doubt, that he has plenty of talent to work with.
"I've not seen Sean Long have too many bad games," Anderson said. "He's a special player, but it was a team effort. It doesn't mean that you don't like people or won't still have a relationship with them, but they are professional rugby league players and that's what they showed tonight."
Anderson was introduced to the Knowsley Road crowd to the strains of: "There's only one Ian Millward," in memory of their sacked coach, but also to warm applause.
But there was a stunned silence when Leeds took the lead through an easy try from the unmarked Marcus Bai after two minutes.
Between that and Leeds scoring again through Jamie Jones-Buchanan just before half-time, Saints controlled the rest of the half.
There were tries from Paul Anderson, Darren Albert and a brilliant solo from Long, who also kicked seven goals from seven attempts.
Vinnie Anderson also got over and James Roby and Long put Ian Hardman in at the start of the second half. With Paul Sculthorpe claiming Saints' final try it was obvious Leeds were on their way to only their second defeat of the season and their first away from home for almost a year.
St Helens: Hardman; Albert, Lyon, Talau, Gardner; Hooper, Long; Fozzard, Cunningham, P Anderson, Gilmour, V Anderson, Sculthorpe. Substitutes used: Bennett, Graham, Roby, Fa'Asavalu.
Leeds: Mathers; Calderwood, McKenna, Senior, Bai; Sinfield, Burrow; Bailey, Dunemann, Ward, Jones-Buchanan, Poching, Ellis. Substitutes used: McGuire, McDermott, Lauitiiti, Botham.
Referee: K Kirkpatrick (Warrington).