The former Celtic striker was on the pitch for less than a minute before he scored the spectacular winner, and he also went close on two other occasions in the second half.
Hamilton worked hard throughout the game and were holding their own up until Riordan's introduction, but they simply didn't have an answer to his skill.
Although Hamilton dominated the second half, with tackles flying everywhere and lots of commitment being shown, they simply didn't do enough to test Yves Ma-Kalambay in the Hibs goal.
Hamilton had the first chance inside two minutes when Brian Easton and James McArthur linked well to set up Simon Mensing, but the ex-Motherwell midfielder's drive whistled inches past the left post.
Easton then saw his tenth-minute shot deflected wide with Ma-Kalambay scrambling and two minutes later Hibs went desperately close to breaking the deadlock when Dean Shiels' corner from the left was headed onto the crossbar by Colin Nish.
In the 37th minute Hibs went close again when Shiels set up Nish, but his low shot was saved to his right by Tomas Cerny.
Riordan climbed off the bench moments later and within 60 seconds he had mugged Mensing on the left and curled a delicate shot past the helpless Cerny and into the top right-hand corner of the net.
Hamilton came out for the second half with their tails up, but it took the introduction of a three-pronged substitution in the 61st minute to really breathe life into their play.
Mark Corcoran, Joel Thomas and Steven Ettien replaced David Graham, Richard Offiong and Sebastien Sorsa, and the trio immediately sparked Hamilton into action.
A neat one-two between James McCarthy and Corcoran in the 64th minute carved open the Hibs defence, but captain Ian Murray recovered well to blast the ball past his own left post for a corner.
Three minutes later Thomas curled a shot just the wrong side of the right post with Ma-Kalambay struggling.
Corcoran went close in the 78th minute when Ma-Kalambay parried his 25-yard pile-driver but James McArthur couldn't react in time as the ball fell at his feet.
Six minutes from time Riordan nearly doubled Hibs' lead when he picked up a pass from fellow substitute Steve Pinau and forced a good save out of Cerny.
Riordan also had the last say in the match, curling a neat free-kick towards the bottom left corner, but Cerny again saved well.