The New Zealand full-back produced a virtuoso performance that inspired the Saints to only their second win of the season with two tries and 20 points.
London Irish led by five points as the clock ticked down but a late tackle from Exiles fly-half Riki Flutey handed Northampton one last chance.
Reihana took the quick tap penalty and the ball was spread quickly to Ben Cohen who jinked through the broken defence for his third try in four games. That drew the scores level at 23-23 and left Reihana with the conversion from wide out on the left touchline to earn Northampton the victory.
It was a frantic end to a roller-coaster game that had appeared to be going the way of London Irish after Flutey marked his first Guinness Premiership start with an 18-point haul.
Northampton had moved 7-0 ahead with Reihana’s third-minute try before Flutey and Dominic Feau’nati ran in long-range scores to turn the game on its head. London Irish led 17-7 but Northampton battled their way back into the game and Reihana emerged the hero some 85 minutes after giving the Saints an ideal start.
Northampton had started brightly with Carlos Spencer’s vision unlocking two quick attacks, the second of which allowed Cohen to slip the ball outside to Reihana who finished off the opening score under the posts.
It took London Irish 10 minutes to engineer their first genuine attack of the game but the Saints’ enterprising endeavour proved their undoing as the Exiles surged into the lead on the back of two Northampton mistakes in the space of
After Flutey had converted a penalty, Spencer ignited a Northampton counter-attack from deep inside their own half. The try looked on with Cohen and Reihana in support – but Flutey gambled on the interception, snatched Spencer’s attempted pass to Reihana and sprinted 55 yards to score under the posts.
Suddenly the Saints, having started with such composure, looked rattled. Reihana took an age to clear from his 22 and his kick was charged down by Ben Willis.
Northampton failed to make decent use of their possession on halfway, allowed the ball to squirt out of the breakdown and Feau’nati set off from inside his own half for another long-range score.
The double blow forced Northampton to tone down their ambition and they rumbled through seven phases before Reihana slotted a penalty. Flutey replied in kind, but Northampton’s low-risk approach saw Spencer decide against launching the long pass to Cohen on the overlap.
Instead the former All Black fed Clarke who shrugged off two attempted tackles on a barnstorming run to the line. When the ball came out for Reihana the Saints full-back sold a dummy outside and cut over for his second try.
The conversion went wide but Reihana landed penalty three minutes before the interval to haul Northampton back within two points at the break.
The Saints enjoyed swathes of possession immediately after the interval but could not forge an opening. Sean Lamont was involved twice in a counter-attack that took Northampton to within inches of the line but an accidental offside released the pressure and London Irish escaped unscathed.
Flutey, having just missed with a penalty attempt, extended London Irish’s lead to 23-18 with 21 minutes remaining. Northampton pressed on again but London Irish smothered Lamont yards from the line and then poached the Saints lineout to clear their lines.
Flutey missed with two simple drop-goal attempts but the points seemed destined for the visitors but London Irish failed to properly clear their lines.
They had plenty of opportunity to do so but could not relieve the pressure and they were punished by Cohen’s last-gasp try and Reihana’s kicking.