Full-back Ben Foden showed his intent with barely 20 seconds on the clock, scything between two tacklers in a counter-attack from his own 22.
After Myler had swiftly cancelled out Jackson's early penalty, another Foden break-out from behind his line in the ninth minute sent Reihana away after a flicked pass from Dylan Hartley.
The full-back then nearly set up the opening try after looping round Reihana in the left corner, only to see the scoring pass inside intercepted by Penney.
Hartley, hoping to make his first England start in the forthcoming series with Argentina, was then sin-binned along with opposing hooker Fabio Ongaro after a dust-up following a scrum.
After another near-miss when Myler's pass eluded Reihana, Saints finally claimed the opening try just before the half-hour.
Prop Sonae Tonga'uiha's barnstorming run from a line-out took him to within a metre of the Sarries line, and some superb handling along the backline sent Reihana over in the left corner, Myler converting.
Bruce Reihana (right) celebrates scoring Saints' opening try
Wing Paul Diggin might have put the game beyond Saracens with a chip and chase in the final move of the first half, but after referral to the video referee, his touchdown was deemed to have been on the dead-ball line, rather than inside it.
Instead the visitors fought back valiantly in the second half, Jackson's second penalty after 52 minutes bringing them to within a converted score.
Foden forced Chris Wyles into touch in the right corner after 67 minutes but Sarries were not to be denied five minutes later, despite Foden initially halting another Wyles burst.
Quickly recyled ball saw Penney cross on the right, Jackson's conversion levelling the scores with seven minutes left.
But Northampton retained their composure, working their way into position for Myler to land a superb drop-goal in the 75th minute.
Sarries were unable to achieve any field position to launch a final attack, leaving Andy Farrell, who came on as a replacement, to contemplate a painful defeat in his final game before retirement.
Northampton director of rugby Jim Mallinder:
"We were excellent in the first half. We played some really good rugby and I thought we deserved our lead at half-time.
"Full credit to Saracens for coming back at us. We defended well and even after they got the try, we came back, got the drop-goal and then looked after the ball well.
"That really showed the maturity in the side and that they are improving. But we won't get carried away. We have a three-week break now but we will come back and work very hard for the final."
Saracens retiring centre Andy Farrell:
"I thought I would be emotional at the end but I am just gutted we didn't win.
"Northampton are a very good side and we were starved of possession in the first half, but we came back and put them under a lot of pressure.
"Since Eddie Jones left five or six weeks ago, a lot of things have been going on and we have become a very tight group. We are all very disappointed."
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