Tony Smith and Kevin Sinfield celebrate another Grand Final win
St Helens: (6) 6
Leeds: (8) 33
Tries: Webb, Lauitiiti, Donald, Smith, Jones-Buchanan
Goals: Sinfield 6
Leeds Rhinos ensured Tony Smith's last game as coach ended with a Grand Final victory over defending champions St Helens at Old Trafford.
The Rhinos led 8-6 at the break after a tough first half in which Brent Webb had put Leeds ahead before James Roby finished a superb break for Saints.
But Leeds scored two tries in as many decisive minutes after the resumption through Ali Lauitiiti and Scott Donald.
Lee Smith and Jamie Jones-Buchanan then scored as Leeds secured a deserved win.
The Grand Final was the third of Tony Smith's career as Leeds coach and the victory ensures he leaves the club with two triumphs at Old Trafford.
It was a deserved victory for the Rhinos, who had finished the regular season second to Saints but were undoubtedly the superior team when it mattered most.
Leeds defended with immense physicality in the opening half and showed a willingness to run the ball when in attack, particularly through Harry Sunderland man of the match winner Rob Burrow.
The Rhinos also showed excellent composure in possession - a facet of the game that was uncharacteristically poor from St Helens, who was ran out of steam after a long and draining season.
Daniel Anderson's team were looking to complete a clean sweep after victory in the World Club Championship, the Challenge Cup and their top spot in the minor Premiership.
And it looked good for Anderson in the opening minutes as Saints started ferociously, throwing the ball wide early in the tackle count and attacking the right side of the Leeds defence.
Long, playing for the first time since 31 August, executed a couple of precise early kicks and should have put Leeds ahead with a penalty from 24 metres after three minutes but pushed his effort wide.
Leeds struggled to get close to the Saints line and did not enjoy any good field position until the 15th minute.
Their first meaningful attack culminated in two points after Chris Flannery was penalised for offside.
In kicking the penalty Sinfield became the first Leeds player to score in every game played by the club in a season.
And having taken the lead, the Rhinos proceeded to score the first try of the final five minutes later.
A smooth passing sequence saw the Leeds backline work the ball towards the left corner. Full-back Webb eventually touched down after a pass from Donald, with Sinfield adding the extras.
Leeds had weathered the early Saints storm and the Rhinos went on to ask further searching questions of St Helens sliding defence - but a high tackle from Danny McGuire relieved the pressure and within minutes Saints had scored.
Long ran through a chink in the Leeds defence and charged downfield before slipping the ball to Lee Gilmour, who in turn fed Man of Steel Roby to cross close to the sticks.
It was developing into a pulsating encounter, with both teams showing a willingness to throw the ball.
Two errors from Sinfield led to a sustained period of Saints pressure, with James Graham stopped just yards short.
Yet before the break the Rhinos enjoyed three consecutive sets and it was the defending champions who had to dig deep to defend their line.
If Leeds were in control going into the break, a series of errors early in the second half allowed Saints back into the tie.
Donald knocked-on at the play-the-ball, as did Clinton Toopi as he attempted to intercept a pass.
Leeds celebrate a deserved win in the Grand Final at Old Trafford
But Saints failed to translate field position into points and were punished when Leeds scored two quick tries.
Lauitiiti bulldozed his way into the corner, throwing dummies to initially wrong-foot the Saints defence and then fending off several St Helens players.
From the following set, Keith Senior slipped the ball to Donald, who burst from inside his own half and brilliantly deceived Paul Wellens before touching down.
Sinfield converted the second try to put his team 18-6 ahead and Rob Burrow slipped a drop-goal between the posts shortly after to extend their lead by a further point.
Saints struggled to mount the period of sustained pressure that would have seen them force their way back into the encounter.
And any hopes of a dramatic comeback were extinguished Smith beat Ade Gardner to a high cross-field kick from Sinfield and crossed for a try.
Sinfield converted and then added a further penalty to push Leeds 27-6 ahead with seven minutes left.
The last meaningful action of an increasingly one-sided encounter saw Jones-Buchanan touch down under the sticks, with Sinfield adding the extras as Leeds beat Saints for the third time in four meetings this season.
St Helens: Wellens, Gardner, Gidley, Talau, Meli, Pryce, Long, Fozzard, Cunningham, Cayless, Gilmour, Flannery, Wilkin.
Replacements: Roby, Bennett, Graham, Fa'asavalu.
Leeds: Webb, Donald, Toopi, Senior, Smith, McGuire, Burrow, Leuluai, Diskin, Peacock, Jones-Buchanan, Ellis, Sinfield.
Replacements: Lauitiiti, Bailey, Kirke, Ablett.
Referee: Ashley Kline (RFL).