Both sides had their moments in an even-handed start to the encounter.
Saints turned on the pressure, a penalty giving them the opportunity to switch the ball to the wing, Gidley’s superb pass sending Ade Gardner over at the corner.
A St Helens break exposed gaps in the Leeds defence, but the final pass from Gidley to Gardner was judged to be forward by referee Phil Bentham.
The Rhinos started the second period with the upper hand, immediately forcing a goal-line drop out, but it was St Helens who got the first points thanks to a penalty goal from Wellens.
John Wilkin, who had a superb game at stand off, kicked a 40/20 – his second – to set Saints up for their first try of the second half, smart handling sending Wellens over the line.
Another try quickly followed, Kieron Cunningham combining with Roby for Gidley to get over the line, the score now 22-6 in St Helens’ favour.
A last-minute try from Senior, Sinfield’s goal kick going in off the post, gave the Leeds contingent hope, but despite a frenetic final few moments Saints had done enough to win.
Leeds – 18
T: Jones-Buchanan (22); Webb (61); Senior (79)
G: Sinfield (22, 61, 79)
St Helens – 22
T: Gardner (14); Roby (28); Wellens (51); Gidley (56)
G: Wellens (28, 47, 65)
Ref: P Bentham