The forward had to be reminded of the milestone he'd just passed, his own personal delight clouded by the fact it wasn't enough to prevent the Easter Road side from tumbling to defeat at Rugby Park.
However, despite that disappointment, Riordan today admitted it was a moment to remember as he immediately set his sights on claiming many more in a green and white jersey.
Already the lifelong Hibs fan is fourth in the list of all-time scorers in the SPL, his 91 league goals bettered only by Kris Boyd, Henrik Larsson and Scott McDonald. But given McDonald, now with Middlesbrough, finished on 93, it seems certain Riordan will soon overhaul that tally while only time will tell whether he can, eventually, better the 158 of Larsson or even Boyd's 164.
However, for Riordan the target is simple, to finish top scorer at Easter Road this season, a title he's virtually made his own over the years.
He said: "I'd actually forgotten my goal at Kilmarnock was my 100th. Obviously as a Hibs fan I am delighted to have scored 100 goals for the club I support - the only disappointment was the final score.
"I don't really set myself any targets, my only aim every season is to finish top scorer. As a striker I feel 20 goals a season is a fair return and having hit 100 I now want to score as many more as I can for Hibs."
Riordan's 25-yard strike at Rugby Park was his seventh goal of the season, the 27-year-old averaging roughly a goal every two games in what, thus far, has been a difficult campaign for the Edinburgh club.
But, he insisted, there is no secret to his success in front of goals - he simply loves scoring, manager after manager at Easter Road having recounted how they've virtually had to drag him off the training pitch.
And, he argued, his shoot-on-sight policy is one more of his team-mates should adopt. He said: "If I get the chance to shoot then I am going to shoot, that's my strength.
"I think it is something we are lacking, we seem to try to walk the ball into the net too much."
While Riordan's approach is simple, opposition teams find it virtually impossible to prevent him getting his shot away from whatever distance and from whatever angle as Killie boss Mixu Paatelainen admitted after watching a player who was just a kid when he was pulling on a green and white shirt reach that milestone.
The big Finn, who was instrumental in bringing Riordan back to Easter Road following a miserable two years with Celtic, said: "We knew exactly what Derek could do, he gets into those positions and he rarely misses."
Riordan can still remember his first goal for Hibs, firing home what was to become a trademark free-kick against Livingston at Easter Road on April 12, 2003, a moment he relived again recently on video.
But, in fact, his first professional goals had come in the unfamiliar colours of Cowdenbeath. Farmed out on loan to Central Park where Skol Cup hero Keith Wright was then manager, the teenager claimed four goals in three games for the Blue Brazil including a hat-trick in a memorable 7-5 victory over Brechin City at Glebe Park.
Riordan, however, was recalled to Easter Road as Spanish striker Paco Luna returned home, his goal against Livingston followed by two more as Hibs sank Aberdeen 3-1 with a certain Scott Brown making his debut. Eighteen goals, including his first derby day strike, came the following season and the next Riordan claimed 23, among them a hat-trick against Kilmarnock.
Twenty more saw Celtic come calling, Riordan, making the ill-fated journey along the M8 where, starved of first team action by boss Gordon Strachan, he managed just seven - one of them against Hibs - in his first season, and only one the next.
But since Paatelainen ensured the prodigal son's return, Riordan has netted 35 more for Hibs, none sweeter than his winner from the penalty spot at Tynecastle after Hibs had crossed the Capital with a makeshift team the season before last having been written off by everyone outwith Easter Road.
Now, though, his aim is to claim plenty more as he bids to help kickstart a season which sees the Edinburgh outfit languishing in ninth place having finished fourth last time round.
Riordan said: "We are right down there and that's disappointing but last season we started so well before falling away after Christmas so hopefully we can do things the other way round this time and start to push ourselves back up the table from now. We have two very important matches against Hamilton tomorrow and Aberdeen on Sunday, chances to put clear water between ourselves and the two teams at the bottom of the table but we must start defending better."
Coming from a Hibs supporting family, Riordan knows only too well the trauma the fans are suffering at present, having seen their side win just four times in 16 SPL matches. He said: "Of course the fans are frustrated, I know what it is like. My family are in the crowd and like everyone else don't want to see us lose. I was the same myself when I was younger and went to games, I always got down when we did not win."
Riordan, however, insisted there remains the confidence within the Easter Road dressingroom that Hibs can make the top six - and ensure a fourth derby of the season against Hearts.
He said: "That's where you want to be, you want to be playing against the best teams and you want another derby - that's the game I look forward to."