Barnsley ended a 10-match run without a victory with a 1-0 win that pushed Sunderland back into the Sky Bet Championship's relegation zone.
Defender Ethan Pinnock was allowed the freedom inside the Black Cats box to head the winner inside two minutes of the second half at the Stadium of Light.
Sunderland, who cannot put a run of victories together to climb away from trouble, had hoped to leapfrog the Tykes after winning at Nottingham Forest on Saturday.
But manager Chris Coleman could only look on in frustration as Barnsley held on to their lead and it was deserved on Wearside.
Since Barnsley boss Paul Heckingbottom was linked with the Sunderland job in November his team have struggled and they were in desperate need of ending their winless run.
In the end Sunderland opted for Coleman - and he has made progress. However, they keep shooting themselves in the foot having claimed three wins in his nine matches, including two draws.
Those victories have given Sunderland a fighting chance of survival but this was another day to forget.
Coleman was forced into changes against the Tykes. Marc Wilson's withdrawal through injury meant full-back Adam Matthews having to play as one of three central defenders, while James Vaughan was asked to lead the line instead of Josh Maja.
Barnsley started the stronger and within 10 minutes striker Mamadou Thiam had crashed an effort against the crossbar from 25 yards, while Adam Hammill saw an effort roll wide.
Sunderland did improve and Callum McManaman had an effort deflect into the arms of goalkeeper Adam Davies, then Bryan Oviedo curled a fine free-kick from 25 yards against the right-hand post.
The best chance of the half fell to Barnsley when Lloyd Isgrove worked his way in behind the Sunderland defence - which lost Darron Gibson's protection in front of it after half an hour through injury - only to be thwarted by the leg of goalkeeper Robbin Ruiter.
Coleman was far from happy so changed the system for the second half, introducing Maja as a second striker in a bid to help the isolated Vaughan. It failed to have the desired effect, as within 98 seconds Barnsley had gone ahead.
Zeki Fryers sent over a deep corner and Matthews - who had been switched to full-back for the second half - was static when Pinnock moved ahead of him to head in the opener.
Barnsley continued to be more dangerous, with Isgrove the dangerman on the flank. After Brad Potts had been denied by Ruiter, Coleman's final move was to bring on Joel Asoro for McManaman and soon after that Paddy McNair volleyed narrowly wide.
After that Barnsley just frustrated the home crowd, with boos regularly directed at the Sunderland players who struggled to make an impression in the final third.
With the exception of a Maja header that dropped over the bar, the visitors were never really in much danger and secured the points.