Riordan pounced six minutes from time with an effort from outside the area that found the net via the far post.
Hibs took the lead through captain Rob Jones in the 14th minute after the defender's glancing header from Riordan's corner beat Mark Howard.
The visitors had chances after the break to level but failed to make their pressure count.
Hibs survived a fourth-minute scare after they failed to clear their lines from Garry Brady's free-kick at the edge of the box.
Stephen McGinn hit a first-time shot from similar range, which struck Jones.
The also visitors conjured up the next opportunity four minutes later, although Dennis Wyness' shot on the turn from an acute angle was straight at Grzegorz Szamotulski.
After gathering Wyness' effort, the goalkeeper's long kick-out created a chance for Riordan on the counter-attack as the striker's left-footed shot from 16 yards was pushed clear by Mark Howard.
Frenchman Steven Thicot went even closer minutes later with a 30 -ard strike that was heading for the top corner until Howard superbly tipped the ball over the bar.
From the resultant set piece, however, Hibs took the lead after Jones' glancing header from Riordan's corner found the net.
The home side were keen to add to their advantage and Lewis Stevenson came within inches of scoring in the 20th minute but, after a superb solo run, his left-footed effort from outside the area just missed the target.
Hibs enjoyed the lions' share of possession, with Sol Bamba in the middle of the park again looking impressive.
Former Dunfermline player Bamba was bypassed, however, as his side set off on another counter-attacking move that ended in Riordan just failing to get a touch on Nish's pass across the six-yard box.
Saints defender Franco Miranda was then alert to clear the danger inside the box as Jonatan Johansson pulled a pass back for John Rankin in a move that began with a marauding run from Riordan.
With five minutes of the first period remaining, Bamba tried his luck from long range near the touchline and the shot forced Howard into a diving save to his left.
A corner four minutes into the second half caused Gus MacPherson's side problems as Bamba rose to meet Riordan's cross but his header bounced wide of goal.
Another set piece from the opposite corner moments later allowed Riordan to deliver an inswinging cross and, although Nish made contact, the towering striker's header sailed over the bar.
Rankin arguably chose the wrong option when he cut out Hugh Murray's stray pass in the middle of the park.
The former Inverness player blasted an effort well wide from just outside the box when both Nish and Riordan had taken up good positions.
Riordan justified his selection with a hard-working performance and he had a shot saved by Howard from 16 yards after being teed up by Nish.
In a rare attack, St Mirren right-back Jack Ross ventured forward and his deflected shot from the edge of the area was well held by Szamotulski.
The Pole made another good save to preserve his side's lead, pushing Scott Cuthbert's back-post header behind from Brady's corner in the 63rd minute.
MacPherson altered his personnel on the flanks by bringing on Billy Mehmet and Steven Robb.
An opening in the Easter Road side's defence allowed Andy Dorman to swivel and shoot but he scooped his attempt at goal well over the bar from 14 yards with a ruck of Hibs players in his path.
St Mirren had Hibs on the back foot in the dying stages and, from Mehmet's cross, Jones headed Dorman's shot off the line.
Any hopes Saints had of drawing level were dashed six minutes from time when Riordan scored his side's second.
Substitute Ross Chisholm fed the forward after a bursting run and Riordan illustrated his striking prowess with a fine finish.
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