Saints, trailing 10-0 after nine minutes, hit back to destroy their visitors with a scintillating eight-try performance which has greatly eased their relegation worries.
Lamont finished with four tries and the others came from flankers Paul Tupai and Sam Harding, England left-wing Ben Cohen and replacement Dylan Hartley. Full-back Bruce Reihana landed six conversions and two penalties for an 18-point contribution.
Saracens were undermined by their mistakes after fly-half Glen Jackson gave them a promising start by landing a third-minute penalty, providing the pass to send winger Dan Scarbrough over in the right-hand corner and adding the conversion.
But then Jackson and half-back partner Kyran Bracken handed Northampton thetwo tries which turned the tables.
Jackson’s attempted clearance kick was charged down by Samoan Tupai for his first try for the club.
Saracens were punished again in the 24th minute when an aimless pass from former England scrum-half Bracken was gratefully accepted by opposite numberMark Robinson who sent Harding over.
Spencer was causing hapless Saracens all sorts of problems and he found space for Cohen but the England star was unable to release the final pass.
Then a scything run by Robinson carved open the Saracens defence and a sustained spell of pressure ended when Cohen appeared on the right wing to run onto a beautifully-flighted kick from Reihana for his side’s third try.
Worse was to follow for Saracens in the second half as Jackson made another glaring error.
He totally misdirected a kick from the halfway line, aimed for team-mate Scarbrough, which flew straight into the arms of Lamont who galloped unchallenged half the length of the pitch for the fourth try which provided Northampton with a valuable and deserved bonus point.
Spencer, revelling in the romp, was turning on the tricks and, facing his own post, he provided one magical moment with an overhead kick which he turned, chased and collected to launch the next Northampton attack which ended with Cohen being forced into touch a yard short of the line.
Then the former All Black provided an inch-perfect cross-field kick for Lamont’s second try in the 54th minute before bursting through to find space and lay on the Scot’s hat-trick try in the 58th.
Flanker Steffon Armitage pulled back a 65th-minute try, converted by Jackson, but it provided only a brief moment of respite.
Hartley responded with Northampton’s sixth try and then Lamont capitalised on yet another Saracens mistake to score his fourth.
Eddie Jones, sacked as Australia’s coach following their autumn defeat by Wales, arrives next month to take up the role of consultant to try to reinforce Saracens’ battle against relegation and, on the evidence of this shambolic performance, he has plenty to do.