In only his second game since rejoining Hibs, the forward demonstrated exactly why boss Mixu Paatelainen was so determined to end his unhappy spell at Celtic and bring him back to Easter Road.
He replaced the injured Steven Fletcher late in the first half and had an immediate impact on the match by helping himself to the decisive goal at New Douglas Park against a team who had enjoyed a decent start to life in the top flight.
Hamilton boss Billy Reid made one change from the side who recorded a surprise win at Aberdeen last week, with Tony Stevenson dropping out of the squad and replaced in the starting line-up by James McCarthy.
Hibs also made a single change following their own win over Dundee United. Sol Bamba missed out through suspension after picking up a red card on his debut and his place was taken by John Rankin.
Simon Mensing tried his luck for Hamilton in the opening couple of minutes with a powerful shot which curled just wide of Yves Ma-Kalambay's right-hand post.
At the other end, a David van Zanten cross was touched onto Dean Shiels by Rankin but he blasted over from distance.
Accies were not too far away when Brian Easton let rip with a shot that seemed harmless enough until taking a wicked deflection off Sebastian Sorsa and falling just wide of the target.
The best chance of the opening quarter fell to Hibs when a Shiels corner found Colin Nish in front of goal and his header crashed off the underside of the crossbar before eventually being gathered by a grateful Tomas Cerny.
The game was crying out for someone to take it by the scruff of the neck and that is exactly what happened.
Fletcher, scorer of both goals in last weekend's win, gestured to the bench that he was in some difficulty and was replaced by Riordan.
And, in a week where Celtic's lack of cutting edge in front of goal cost them a win in the Champions League against Aalborg, the man who failed to make his mark at Parkhead opened the scoring after less than a minute on the park.
Ian Murray was the provider with a cross out to the left-flank and Riordan feigned a move to his left before sending a superb, curling right-foot shot past Cerny with 39 minutes on the clock.
Van Zanten, Sorsa and Mensing were all booked early in the second half as the match grew more heated.
Hamilton made a dramatic triple-substitution in a bid to force themselves back into the game, with Stephen Ettien, Joel Thomas and Mark Corcoran all introduced to the action on the hour mark.
It was Murray who almost inadvertently provided an unlikely leveller though, narrowly avoiding stabbing a Corcoran cross intended for McCarthy into his own net.
Thomas then forced Ma-Kalambay into an impressive save with a ferocious shot from the edge of the box as Accies attempted to pile on the pressure.
The hosts could have restored parity with just over 10 minutes to go when a thunderous long-ranger from Corcoran was parried by Ma-Kalambay and James McArthur blasted the rebound agonisingly wide.
Riordan tried to kill off any hope the home side had of a comeback but he was denied his second of the day when his low shot was just tipped past the upright by a fully-stretched Cerny.
Hibs were then happy to run down the clock as the Edinburgh club held on for the win.