Mendis (3/9) ran through New Zealand's middle order to trigger a batting collapse as the Kiwis, in their pursuit of a 159-run victory target, were bundled out for 110 runs in 17 overs.
It was an abject batting capitulation by the New Zealand with just three batsmen; Martin Guptill (43 off 34 balls), Aaron Redmond (23) and Brendon McCullum (10) -- managing double figures as their last eight wickets fell for just 48 runs.
Earlier opting to bat, Mahela Jayawardene struck an unbeaten 41 while Tillakaratne Dilshan's made a classy 48 to help Sri Lanka post 158 for five on the board.
Defending the total, Sri Lankan skipper Kumar Sangakkara's experiment to start with Sanath Jayasuriya did not really click as Redmond got New Zealand off to a flier, picking up 14 runs of the third over.
Redmond clobbered Jayasuriya for three successive boundaries and then followed it up with a six over long-off.
But Isuru Udana struck gold in the next over, getting the crucial wicket of McCullum, caught by Jehan Mubarak.
Lasith Malinga then foxed dangerous Redmond with a slower full toss and Chamara Silva made no mistake to end the right-hander's 13-ball stay. Mendis then proved why he is regarded so highly in world cricket, getting the wickets of Ross Taylor (8) and Scott Styris (2) in his opening over to deal twin blows.
He foxed Taylor with his first ball of the match and Sangakkara completed an easy stumping and then disturbed Styris' timber with the fourth delivery.
Guptill, however, carried on the fight single-handedly and hit Dilshan over the short long-off fence.
Udana struck in the last ball off the 13th over, clean bowling Jacob Oram and then Jayasuriya drived the final nail in New Zealand's coffin, getting the vital wicket of Guptil caught by Angelo Mathews at deep backward square leg.
Mendis picked up his third wicket when he made Peter McGlashan to edge one straight to Mubarak at backward point.
As if it was not enough, Nathan McCullum and Kyle Mills ran themselves out and then Daniel Vettori fell to Muttiah Muralitharan.
Earlier electing to bat, Sri Lanka made a disastrous start to their innings, losing the dangerous Jayasuriya in the very first over.
Vettori's ploy to start the proceedings with off-spinner Nathan McCullum worked wonders as Jayasuriya feel to the trap, top edging a sweep to Ross Taylor at short fine leg.
Next man in Chamara Silva hit Mills for two boundaries before perishing, caught at mid-on by Jacob Oram off the pacer's second over. Sangakkara and in-form Dilshan then shared a 47-ball 62-run fourth wicket stand to bring back the Lankan innings on track.
Dilshan was going great guns and struck Ian Butler for three boundaries in the sixth over. He struck five fours in his 48-run innings that came off 37 deliveries before Vettori outsmarted him in the 12th over.
After Dilshan's dismissal, Sangakkara took the onus of guiding the Sri Lanka innings in his own hands and played the role of a sheet anchor as new man Jayawardene donned the hat of aggressor.
Jayawardene made it clear that he was not there to hang around and lofted Vettori straight over his head off a free-hit delivery to mark his arrival. He hit Mills for three consecutive fours in the 16th over to up the tempo.
When it looked the duo would ensure a big score for Sri Lanka, Vettori once again broke the dangerous looking partnership, picking up Sangakkara but not before the Lankan skipper had made a useful run-a-ball 35 that was studded with three fours.
Vettori was the pick of the Kiwi bowlers with two wickets, while Butler, Nathan McCullum and Mills scalped a wicket