Reihana's brace takes his tally for the season to 12 – he is now the Premiership's leading try-scorer, two ahead of Sale Sharks wing Steve Hanley, who scored a hat-trick against Gloucester on Friday evening.
Reihana – who won the Zurich Player of the Month award for February – was joined on the scoresheet by Australian fly-half Shane Drahm who kicked 11 points, and Darren Fox who also crossed for a try.
Saracens, without 11 senior players, could muster only a first-half penalty from Thomas Castaignède in reply.
The Frenchman's points actually opened the scoring in the sixth minute – but from that point on it was all one-way traffic.
Chaos descended on the visitor's after key man Castaignède was forced to depart with a leg injury.
His replacement, debutant Matt Honeyben, also fell victim to an injury, forcing former England scrum-half Kyran Bracken to see out the game in the unfamiliar position of fly-half.
Drahm equalised from in front of the posts in the 23rd minute, before Reihana sprinted in from 50 metres just before the break.
Fox went over from close range in the 47th minute and Reihana's second try on 67 minutes clinched the win for Northampton.
The loss leaves Saracens stranded in 10th position on the table. Their recently-purchased band of reinforcements just can't arrive soon enough …
For Northampton Saints:Tries: Reihana 2, FoxCons: Drahm 3Pen: Drahm
For Saracens:Pen: Castaignède