Watford gained three valuable points in their effort to put a little fat around their position as they face a testing series of fixtures in the run up to Christmas.
"We've had a six-game unbeaten run and after the midweek defeat we have bounced back," said Watford boss Ray Lewington "We face some tough fixtures but we must take some encouragement from the fact that we have only lost one in the last eight games.
"Our problem is killing off teams when we are on top. We had loads of chances but the second goal proved elusive and we had a nervous last ten minutes." Danny Webber, the subject of a £450,000 bid from Nottingham Forest in midweek, scored the winner and could have bagged a hat-trick.
Just before he found the net he broke clear onto a pass from Wayne Brown only to be denied by United keeper Gary Montgomery.
"We switched off after that save and got caught," said Rotherham boss Ronnie Moore who blamed his back four for the defeat.
"Right across the back we were run ragged and we didn't deserve anything from this game. To say I am disappointed is to put it mildly." Neither team impressed in the first half although Watford had a couple of scares, first when captain Neil Cox appeared to bring down the slipping Darren Byfield in the box and then when Richard Barker fired in a shot that Neal Ardley deflected just past the post.
Watford went close with a couple of attempts from Scott Fitzgerald but after the interval they opened up the visitors defence on a succession of occasions.
Webber gave them the lead after 46 minutes, rounding off an Ardley pass with a right-footed shot and then Montgomery tipped over following a free kick from Ardley.
Chris Sedgwick cleared off the line when Cox came in with a flying header and then Webber broke clear again only to be outfoxed by Montgomery.
Twice Fitzgerald was in a good position to increase the lead but failed to find the target.
Rotherham had one great chance of an equaliser when Barker found himself free at the far post but he volleyed straight at keeper Lenny Pidgeley.