It was far from the dream return to Upton Park that Sam Allardyce had in mind.
Michail Antonio's goal in the 30th minute was enough to seal all three points for Slaven Bilic's Hammers side as they maintained their push for European football.
Sunderland had more than enough chances to get something out of the game, no question.
But Jermain Defoe, Jack Rodwell and Patrick van Aanholt - at fault for the goal - all squandered glorious chances.
Here's what we learned from the defeat:
Wayward finishing cost Sunderland dear:
Patrick van Aanholt was at fault for the West Ham winner, failing to clear the ball initially before allowing Antonio to then seamlessly ghost past him.
But it was Sunderland's wayward finishing that ultimately cost them dear.
The Black Cats, who hit the bar through Wahbi Khazri, had enough chances in the second half to come away with at least a point.
Defoe flashed a half-volley wide from a great position, Van Aanholt blasted over while Jack Rodwell had one goal harshly disallowed but missed other great openings.
West Ham twice hit the crossbar but Sunderland's poor finishing cost them in the end.
There's a time and a place for showboating:
And that time is when you are three or four nil up and coasting towards victory.
Not when your side is battling relegation and every minute, every moment matters.
What Wahbi Khazri was doing in the final stages of the game when he opted for a rabona cross instead of your more standard delivery, we'll never know.
But instead of the ball being played into the box from a dangerous position, the ball trickled out of play instead as he miscued his attempted cross.
Allardyce's face was a picture. He decided against commenting on it post-match. A wise move. Unlike Khazri's.
Jan Kirchhoff ran the midfield:
The January signing endured a nightmare debut the last time Sunderland played a London side on a Saturday lunchtime.
But ever since that horror-show at Spurs, Kirchhoff has quickly managed to change opinions about the Bayern Munich recruit.
He excelled against Manchester City, was steady at Liverpool before pulling up with a hamstring injury early on against Manchester United.
But he was back to his best against West Ham, bossing the midfield, winning his tackles, headers and generally dominating the middle of the park.
He always looks for a forward pass too. His performance a major plus from the defeat at Upton Park.
Crystal Palace game is now must-win:
Allardyce had targeted four points from West Ham, Palace and Southampton this week - eight days that are set to define the season.
Well, one game down and that target is still achievable.
But defeat in East London means the home game with Alan Pardew's struggling Palace side on Tuesday night is a must-win.
They have already beaten them once this season, Jermain Defoe's winner enough to bring all three points back from Selhurst Park.
With a daunting trip to Southampton to end the week, Tuesday's match presents the best opportunity for a win.
And with just 11 games now remaining, time is quickly running out.
Sam Allardyce's West Ham return a non-event:
Allardyce's return to his old stomping ground had dominated the news agenda building up to the game.
But in the end he was probably left wondering what the fuss was all about as he very much went under the radar.
There was muted applause when he made his way to the dugout to take his seat, nodding, smiling and waving to the Hammers fans closest to him.
He even signed autographs for home supporters, while he enjoyed a bit of touchline banter with one of his signings; Aaron Cresswell, during the game.
There was no obvious abuse from the home support, who had fallen out of love with Big Sam towards the end of his four-year stay.
Instead, they were left cheering another home win with the Hammers now unbeaten on home soil since August.
Sunderland kept Dimitri Payet quiet:
Sunderland, it has to be said, defended well at Upton Park.
But one error, one shocking piece of defending from Patrick van Aanholt after 30 minutes cost them dear.
And at this level, one mistake can cost you all three points.
That's why defeat was doubly frustrating for Allardyce as his side had managed to nullify the threat posed by the Hammers attack - including one of this season's star players in Payet.
The Frenchman was kept quiet for the majority of the game, with Jan Kirchhoff, John O'Shea and Lamine Kone again defending resolutely.