Just when it had looked like West Ham's dogged defending was going to win the day, the England striker pounced for the Black Cats to net his seventh goal of the season.
The defeat saw Hammers slump to 16th place - just five points off the drop zone, leaving Glenn Roeder to reflect on a string of costly misses either side of the break
"The result was more important than the performance today," conceded Peter Reid after seeing his side clinch that late win. "West Ham were a little bit unlucky and we rode our luck."
While Sunderland had been unchanged following last week's defeat at Liverpool, Roeder was forced to make one change following Hammers' defeat against Tottenham Hotspur, as Michael Carrick shrugged off his hamstring strain to replace luckless skipper Steve Lomas (broken toe).
The mercilessly jeered Don Hutchison was handed the captain's armband as he took his place in a 4-4-2 line-up to face his former club for the first time since his £5million return to Upton Park in September.
Hutchison wasted no time showing where his loyalty now lies with a string of enthusiastic early lunges at his former team-mates, while David James' equally energetic rush from goal on six minutes saw him bravely pluck the ball off the breaking Kevin Phillips' toes.
Although a groin strain forced Reid to replace the injured Jason McAteer with Thomas Butler as early as the 17th minute, the switch did not disturb his mirroring 4-4-2 formation as the Black Cats predictably launched everything towards their twin strike-force of Phillips and Niall Quinn.
Midway through the half however, West Ham finally mustered their first effort on goal as Jermain Defoe robbed Bernt Haas and curled well wide of the angle from 20 yards, and then Joe Cole cut inside and drilled a firm shot into Thomas Sorensen's midriff from similar range.
Both sides' approach play was, by now, becoming more and more disjointed in an increasingly niggly game and after Phillips met Quinn's nod down with an 18-yard volley which James held at full stretch, referee Peter Jones finally lost patience and booked Paul Thirlwell for tripping Cole.
Five minutes before the break, West Ham should have broken the deadlock when James' clearing punch allowed Sinclair to release Defoe from halfway but unfortunately the teenage-striker fired his low, angled 12-yarder across the face of the exposed Sorensen and beyond the far post.
Following that tepid opening 45 minutes, West Ham made an explosive start to the second period. Within moments of the restart Joe Cole, seemingly adopting a shoot-on-sight policy, forced Sorensen to acrobatically palm his 18-yard shot skywards, and seconds later Hammers were again denied the lead as the Sunderland 'keeper parried Defoe's shot, before tipping Carrick's follow up onto the crossbar.
The lure of a goal to silence the boo-boys then proved too irresistible for Hutchison who, to the disgust of his team-mates, selfishly screwed across goal from an almost impossible angle on 49 minutes.
Having seen his side come so unexpectedly under the cosh, Reid then replaced Quinn with Lilian Laslandes, and midway through the half, the supporting Haas fired past the far post to add to the frustration of the home fans becoming increasingly impatient at their side's inability to breach a determined West Ham defence.
With just a quarter-of-an-hour left, Thirlwell's low 20-yarder was well saved by James as it scorched through a crowd of bodies, and after Phillips' almost bundled home, Lalsandes' snap-shot was superbly held by the Hammers' 'keeper.
For once, substitute John Moncur - replacing the tiring Cole - was on the receiving end of an unsightly lunge which resulted in a yellow card for Haas, who joined the earlier dissenting Butler in the book.
By now, the unyielding Hammers looked set for a hard-earned point, but with just six minutes left, Julio Arca's low cross into the box was met by the predatory Phillips who beat the dive of James by a split-second to bundle home the winner from ten yards.
Minute-by-minute:
1 - Carrick's long pass sets Defoe away on the left, but the young striker is thwarted by a good tackle from Thome.
17 - Substitution for Sunderland, Butler replaces McAteer.
24 - Defoe does well to dispossess Haas and then wriggles into the area before firing in a rising shot that clears the crossbar.
32 - Quinn's knockdown reaches Phillips, whose snapshot from the edge of the area is well held by James.
34 - Thirwell booked for foul on Cole.
35 - Defoe rises well to challenge Thome and Cole collects the loose ball before beating Williams and firing in a dipping shot that is held by Sorensen.
40 - Hammers break quickly from a Sunderland corner and Sinclair's pass releases Defoe, who times his run perfectly but then fires the best chance of the match so far just inches wide of the target.
42 - Sinclair combines brilliantly with Schemmel on the right after good work by Hutchison, but the resulting cross is cleared for a corner.
Added time: 1 minute.
Half-time: Sunderland 0 West Ham United 0.
Second half:
47 - Cole runs at the Sunderland defence and fires in a blistering shot that is palmed away by Sorensen.
48 - Cole is once again in the thick of things as he advances towards the area and slips a pass into Defoe, whose shot is blocked by Sorensen. The rebound falls kindly to Carrick, whose effort appears to be goalbound until Sorensen tips it on to the crossbar and out for a corner.
49 - With Hammers in the ascendancy, Hutchison wriggles free in the box but fires his shot wide of the target.
51 - After Schemmel is penalised for handball, Phillips curls a free-kick just over the bar from 25 yards out.
57 - Substitution for Sunderland, Laslandes replaces Quinn.
69 - Haas fires in a low shot that fails to trouble James.
70 - Butler booked for failing to retreat 10 yards at a free-kick.
77 - Sinclair's ball over the top once again beats the offside trap and Defoe races clear only to be denied by a last ditch tackle from Haas.
78 - After a cross is only half cleared, Thirwell's fierce low drive is expertly held by James.
79 - Substitution for West Ham, Moncur replaces Cole.
80 - A long ball over the defence isn't dealt with by James and Phillips looks set to score until Repka bravely clears from under his own crossbar. Arca knocks in the loose ball to Laslandes, whose hooked shot is gathered by James.
83 - Haas booked for foul on Moncur.
85 - GOAL. Disaster strikes as Arca skips away from Sinclair and whips in a dangerous low cross that Phillips reaches just ahead of James and pokes into the net. Sunderland 1 West Ham 0.
Added-time: 3 minutes.
90 - Substitution for West Ham, Todorov replaces Minto.
Full-time: Sunderland 1 West Ham United 0.