Despite a bright start, the Black Cats fell a goal down in the 17th minute when defender Christian Kabesle headed home a corner.
Sunderland were then undone by another set-piece, as Ryan Porteous headed in another corner midway through the second half.
The hosts did manage to pull a goal back immediately when Luke O’Nien converted from close range in the 70th minute, before Patrick Roberts scored an excellent equaliser in stoppage-time.
Here’s how Phil Smith rated the Sunderland players against Watford:
5. Dennis Cirkin - 6
Got into some really good attacking areas in the first half and played one or two dangerous crosses throughout. Steady defensively. 6 Photo: Frank Reid
6. Dan Neil - 5
One of many who couldn’t find their sharpest and most incisive passing game on what turned to be another frustrating afternoon at home. 5 Photo: Martin Swinney
7. Pierre Ekwah - 6
Started really well, snapping into challenges and breaking up some dangerous Watford attacks. Kabasele’s goal changed the complexion of the game and so Mowbray had to try and get Pritchard on early in the second half to add an attacking threat. 6 Photo: Frank Reid
8. Patrick Roberts - 7
Had endured a frustrating afternoon with a couple of efforts going close but his last-gasp goal was a real beauty, arched into the far corner and leaving the goalkeeper with no chance. Another great moment in a fine season. 7 Photo: Martin Swinney