Scrum-half Jonny Lomax scored two of his side's five tries as Saints bounced back from their Challenge Cup exit at the hands of arch rivals Wigan to inflict more misery on their 2011 Grand Final conquerors.
Leeds twice held the lead as they attempted to avenge their 46-6 defeat at Langtree Park in March, but St Helens produced a strong finish to secure the points and leave their rivals trailing in eighth place at the half-way stage of the regular season.
The champions made the brighter start with full-back Brent Webb, making his 150th start for the club, producing an arcing run to get winger Ryan Hall charging down the wing and stand-off Danny McGuire was on his inside to take the pass and score the opening try after 11 minutes.
Saints drew level when Lance Hohaia's kick toward the Leeds line re-bounded off defender Lee Smith into the grateful arms of Lomax, who forced his way over near the posts for his first try.
A superb Tony Puletua offload got James Roby into space and Lomax was in support to get skipper Paul Wellens over. Tommy Makinson added both conversions to put his side 12-6 in front, but Sinfield dragged his side level just before half-time when broke through the first line of defence to send second rower Brett Delaney over for a second try.
The Yorkshiremen looked to have put themselves in the driving seat when second rower Jamie Jones-Buchanan took Webb's pass and went between two defenders for a third try seven minutes into the second half and Sinfield's third goal restored their six-point advantage.
But Saints roared back with two tries in eight minutes to go back in front. Makinson took Michael Shenton's one-handed pass to produce a wonderful finish, surviving a shoulder charge by Webb to plant the ball down one-handed by the corner flag and Lomax took a return pass from Sia Soliola to score his second try.
Makinson kicked his third goal to make it 22-18 but it proved to be his last contribution to the game. Play was held up for five minutes so that Makinson could be treated for a head injury before being carried off on a stretcher and Sinfield was placed on report for the challenge that appeared to knock him unconscious.
Saints got the penalty from the incident and set up an attacking position from which they scored the match winning try, with loose forward Jon Wilkin taking Roby's pass to go through a hole in the Leeds defence. Lomax added the conversion and put the seal on a superb win with a penalty and a drop goal to put the visitors out of sight.