Former Widnes and Wigan playmaker Long, who went off injured moments after surpassing the milestone, scored two tries and five conversions as the Centurions - whose season was starting to take shape after successive victories in the league and Challenge Cup - wilted under a barrage of Saints pressure.
Australian centre Jamie Lyon helped himself to three scores while Ade Gardner (2), Richard Wellens, Vinnie Anderson and Jason Hooper (2) weighed in with the others as the Saints just failed to leapfrog the Rhinos at the summit.
St Helens, particularly in the first half, displayed all the nonchalant passing skills and telling lines of running that have been their trademark so far this season.
But they will be disappointed with their showing in the second period as the league?s basement side battled back and registered consolation tries through Steve Maden and John Wilshere (2).
The pain began for Leigh in the second minute when Lyon breezed through two tackles to ground.
Three minutes later, Long galloped over from 20 yards following a short offload from ubiquitous Great Britain hooker Keiron Cunningham.
Lyon doubled his personal tally for the evening after 13 minutes when he side-stepped his marker on halfway and glided over down the left channel.
Wellens brushed aside two would-be challengers to go over in the 24th minute before James Roby bulldozed through some weak tackling from the restart to send Long over from 40 yards.
The try meant the scrum-half had passed the 2000-point barrier - and for good measure he knocked over the conversion, his fifth of the night, to make it 30-0.
Long went off to receive treatment on his knee - and did not reappear - but the scoring for the half did not stop there.
Winger Gardner gleefully bounded over following great work by Roby, with Lyon - the kicker in the absence of Long - adding the extras.
Anderson scurried over in the corner for Saints? seventh try before Hooper, returning after seven months out with knee trouble, got in an the act to round off the first-half scoring on the hooter.
The second period began in much the same vein with Gardner sprinting over majestically from inside his own half in the 42nd minute to bring up the half-century and the ebullient Lyon grabbing his hat-trick three minutes later.
But Leigh finally showed some resolve and Maden managed to nick a try in the 51st minute and Wilshere added a second on the hour mark.
Gardner became the second Saint to hobble off and worse was to follow for the hosts when Papua New Guinea international Wilshere raced over from 20 yards to notch his second of the evening.
The returning Hooper had the last word, however, intercepting a long Centurion pass to gallop over for the final try of the night. St Helens (46) 60 Tries: Lyon (3), Long (2), Wellens, Gardner (2), V Anderson, Hooper (2) Goals: Long (5), Lyon (5) Leigh (0) 16 Tries: Maden, J Duffy, Wilshere Goals: Wilshere (2) St Helens: Wellens, Hardman, Hooper, Gardner, Albert, Lyon, Long, Fozzard, Cunningham, Mason, V. Anderson, Gilmour, Roby. Replacements: Higham, Fa'asavalu, Graham, Bennett. Leigh: Wilshere, Maden, Cooper, Jackson, Smyth, Kent, J. Duffy, Sturm, Govin, Fleary, Stapleton, Leafa, Knott. Replacements: Taylor, Maddox, Marshall, King. Referee: Karl Kirkpatrick (Warrington)
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