The Hammers' substitute struck in the 89th minute to further ease the pressure on his manager, Alan Pardew, whose celebrations at recording unlikely back to back victories, also saw him embroiled in a surreal pushing and shoving match with Arsene Wenger.
West Ham's victory over Blackburn Rovers a week earlier may have ended a record of eight successive league and cup defeats, but they had still kicked off without a clean sheet all season and in 18th place.
And despite being last Sunday's Rovers hero, Teddy Sheringham found his 40-year-old legs rested alongside George McCartney, as Pardew recalled Paul Konchesky and Lee Bowyer to a fixture which Hammers had not won since October 1999.
Beaten just twice in 15 league and cup matches, fifth-placed Arsenal, on the other hand, were unchanged from their midweek Champions League draw with CSKA Moscow, as they looked to extend their undefeated Premiership run to eight games.
Notwithstanding the wild contrast in their early-season fortunes, West Ham emerged confident and composed as man-of-the-match Danny Gabbidon let fly with an early shot, before Matthew Etherington's angled, touchline screamer into the danger zone was deflected past his far post by Gael Clichy.
With Arsenal stretching the Hammers with a delightful range and repertoire of precision passing, however, the eastenders knew that they switched off at their peril and, on 12 minutes, only the outstretched left palm of Robert Green denied Robin Van Persie after he latched on to Thierry Henry's subtle nod down from Alex Hleb's searching cross.
In reply, Yossi Benayoun headed Jonathan Spector's cross behind at the near post, but it was Tomas Rosicky who should have broken the deadlock on the half-hour mark, when Cesc Fabregas and Henry carved West Ham apart only for the Czech to wastefully drill wide of the exposed Hammers' keeper and his left-hand upright.
Just before the break, Van Persie -who had earlier been felled by a missile from the crowd - followed Clichy into referee Styles' notebook for a deliberate handball and that capped an eventful 45 minutes for the duelling Dutchman, who had been involved several scraps with Spector, who likewise had also been cautioned for an early scythe on the Gunner.
On the hour, Van Persie trudged away after Emmanuel Adebayor was asked by a frustrated Wenger to increase the pressure on a West Ham defence that had lost Anton Ferdinand just before the break.
With his side still very much in an intriguing contest, though, Pardew responded by replacing the industrious Bowyer and Bobby Zamora with Harewood and Sheringham and, within minutes of his arrival, only the courageous Clichy's last-gasp block thwarted the former Tottenham veteran.
At the other end, Hleb was denied a penalty when he was brought down by Spector and, when the hitherto subdued Henry finally mustered a shot on target, Green got down well to grab the 20-yard effort, before Kolo Toure was just inches wide from double the distance.
With a quarter-hour left, Harewood looked set to break the deadlock when he raced clear with only Lehmann to beat, however the giant German cleverly tightened the angle to block the shot.
But with the clock ticking down, Konchesky asked Etherington to embark on one last touchline gallop and when the Hammers' winger crossed into the six-yard box, Harewood atoned for that earlier miss when he swept home at the far post to send the East End crazy and Pardew and Wenger even wilder.